Transcriber Note Text emphasis denoted by _Italics_ and =Bold.= Whole and fractional parts of numbers as 123-4/5. THE ANCIENT VOLCANOES OF GREAT BRITAIN [Illustration] THE ANCIENT VOLCANOES OF GREAT BRITAIN BY SIR ARCHIBALD GEIKIE, F.R.S. D.C.L. Oxf., D.Sc. Camb., Dubl.; LL.D. St. And. Edinb. DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN AND IRELAND; CORRESPONDENT OF THE INSTITUTE OF FRANCE; OF THE ACADEMIES OF BERLIN, VIENNA, MUNICH, TURIN, BELGIUM, STOCKHOLM, GÖTTINGEN, NEW YORK; OF THE IMPERIAL MINERALOGICAL SOCIETY AND SOCIETY OF NATURALISTS ST. PETERSBURG; NATURAL HISTORY SOCIETY, MOSCOW; SCIENTIFIC SOCIETY, CHRISTIANIA; AMERICAN PHILOSOPHICAL SOCIETY; OF THE GEOLOGICAL SOCIETIES OF LONDON, FRANCE, BELGIUM, STOCKHOLM, ETC. WITH SEVEN MAPS AND NUMEROUS ILLUSTRATIONS IN TWO VOLUMES VOL. II London MACMILLAN AND CO., Limited NEW YORK: THE MACMILLAN COMPANY 1897 _All rights reserved_ CONTENTS CHAPTER XXIX The Carboniferous Volcanoes of England PAGE The North of England: Dykes, Great Whin Sill--The Derbyshire Toadstones--The Isle of Man--East Somerset--Devonshire 1 CHAPTER XXX The Carboniferous Volcanoes of Ireland King's County--The Limerick Basin--The Volcanic Breccias of Doubtful Age in County Cork 37 BOOK VII THE PERMIAN VOLCANOES CHAPTER XXXI The Permian Volcanoes of Scotland Geographical Changes at the Close of the Carboniferous Period--Land and Inland-Seas of Permian time--General Characteristics and Nature of the Materials erupted--Structure of the several Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale; 2. Basin of the Firth of Forth 53 CHAPTER XXXII Permian Volcanoes of England The Devonshire Centre--Eruptive Rocks of the Midland Coal-fields 94 BOOK VIII THE VOLCANOES OF TERTIARY TIME CHAPTER XXXIII Vast lapse of time between the close of the Palæozoic and beginning of the Tertiary Volcanic Eruptions--Prolonged Volcanic Quiescence--Progress of Investigation among the Tertiary Volcanic Series of Britain 107 CHAPTER XXXIV The System of Dykes in the Tertiary Volcanic Series Geographical Distribution--Two Types of Protrusion--Nature of component Rocks--Hade--Breadth--Interruptions of Lateral Continuity--Length--Persistence of Mineral Characters 118 CHAPTER XXXV The System of Dykes--_continued_ Direction--Termination upward--Known vertical extension--Evidence as to the movement of the Molten Rock in the Fissures--Branches and Veins--Connection of Dykes with Intrusive Sheets--Intersection of Dykes--Dykes of more than one infilling--Contact metamorphism of the Dykes--Relation of the Dykes to the Geological Structure of the Districts which they traverse--Data for estimating the Geological Age of the Dykes--Origin and History of the Dykes 145 CHAPTER XXXVI The Plateaux Nature and Arrangement of the Rocks: 1. Lavas.--Basalts, Dolerites, Andesites--Structure of the Lavas in the Field--2. Fragmental Rocks.--Agglomerates, Conglomerates, and Breccias--Tuffs and their accompaniments 181 CHAPTER XXXVII The Several Basalt-Plateaux and their Geological History--Antrim, Mull, Morven and Ardnamurchan 199 CHAPTER XXXVIII The Basalt-Plateau of the Parish of Small Isles--Rivers of the Volcanic Period 215 CHAPTER XXXIX The Basalt-Plateaux of Skye and of the Faroe Isles 249 CHAPTER XL The Modern Volcanoes of Iceland as illustrative of the Tertiary Volcanic History of North-Western Europe 260 CHAPTER XLI The Eruptive Vents of the Basalt-Plateaux Vents filled with Basalt or other Lava-form Rock--Vents filled with Agglomerate 270 CHAPTER XLII The Basic Sills of the Basalt-Plateaux 298 CHAPTER XLIII The Bosses and Sheets of Gabbro Petrography of the Rocks--Relations of the Gabbros to the other members of the Volcanic series--Description of the Gabbro districts--Skye 327 CHAPTER XLIV The Bosses and Sheets of Gabbro in the Districts of Rum, Ardnamurchan, Mull, St. Kilda and North-East Ireland. History of the Gabbro Intrusions 349 CHAPTER XLV The Acid Rocks Their Petrography--Their Stratigraphical Position and its Analogies in Central France 364 CHAPTER XLVI Types of Structure in the Acid Rocks--Bosses 378 CHAPTER XLVII The Acid Bosses of Mull, Small Isles, St. Kilda, Arran, and the North-East of Ireland 395 CHAPTER XLVIII The Acid Sills, Dykes and Veins 430 CHAPTER XLIX The Subsidences and Dislocations of the Plateaux 447 CHAPTER L Effects of Denudation 455 CHAPTER LI Summary and General Deductions 466 LIST OF ILLUSTRATIONS FIG. PAGE 176. Section from the great Limestone escarpment on the west to the Millstone Grit hills east of Teesdale 4 177. Sections of the Carboniferous Limestone series of Northumberland showing the variations in the position of the Whin Sill. By Messrs. Topley and Lebour 6 178. View of two volcanic necks in the Carboniferous Limestone series, at Grange Mill, five miles west of Matlock Bath, from the north 14 179. Plan of necks and bedded tuff at Grange Mill, five miles west of Matlock Bath 15 180. Section across the smaller volcanic neck and the stratified tuff in Carboniferous Limestone, Grange Mill 15 181. Section of vesicular and amygdaloidal diabase resting on Carboniferous Limestone, Peak Forest Limeworks, Great Rocks Quarry 19 182. View of the superposition of Carboniferous Limestone upon toadstone, Raven's Tor, Millersdale (length about 100 feet) 19 183. Section at lime-kiln, south of Viaduct, Millersdale Station 20 184. Limestones passing under stratified tuffs, Poyll Vaaish, Isle of Man 24 185. Section of tuff, showing intercalations of black impure chert, west of Closenychollagh Point, near Castletown, Isle of Man 25 186. Section of intercalated dark limestone, shale and chert in the tuff south of Poyll Vaaish Bay, Isle of Man 26 187. Section of part of a volcanic neck on shore to the south-east of Poyll Vaaish Bay, Isle of Man 29 188. Section of successive discharges and disturbances within a volcanic vent. Scarlet Point, Isle of Man 29 189. Section of dyke and sill in the tuffs west of Scarlet Point, Isle of Man 30 190. Section on south side of vesicular sill west of Scarlet Point 31 191. Bands of vesicles in the same sill 31 192. Croghan Hill, King's County, from S.S.W. 38 193. Section in quarry on roadside east of Limerick, close to viaduct of the Limerick and Erris Railway 44 194. Section of the volcanic escarpment, east of Shehan's Cross-roads, south of Limerick 45 195. View of Derk Hill, a volcanic neck on the south side of the Limerick basin 47 196. Section across the Limerick volcanic basin 48 197. Section of a bed of volcanic breccia in the Carboniferous Slate; White Bull Head, County Cork 50 198. Volcanic breccia invading and enclosing Carboniferous slate, White Bull Head 50 199. General section across the Permian basin of Ayrshire 59 200. Section of lavas, east side of Mauchline Hill 60 201. Section of the top of the volcanic series near Eastside Cottage, Carron Water, Nithsdale 60 202. Section of two outliers of the Permian volcanic series at the foot of Windyhill Burn, Water of Ae, Dumfriesshire 61 203. The Green Hill, Waterside, Dalmellington, from the south; a tuff-neck of Permian age 62 204. Patna Hill from the Doon Bridge, Ayrshire; a tuff-neck of Permian age 63 205. Ground plans of Permian volcanic vents from the Ayrshire Coal-field. On the scale of six inches to a mile 64 206. Section of sills traversing the Permian volcanic series. River Ayr, Ballochmyle 66 207. Section showing the relations of the later rocks of Arthur Seat 68 208. Section in brooks between Bonny town and Baldastard, Largo 70 209. View of Largo Law from the east 71 210. View of small neck in Calciferous Sandstones, on the shore, three miles east from St. Andrews 72 211. Ground-plan of Permian volcanic vents 73 212. Small neck in Calciferous Sandstones a little east from the "Rock and Spindle," two and a half miles east from St. Andrews 74 213. Plan of volcanic necks at Kellie Law, East of Fife, on the scale of three inches to one mile 75 214. Plan of the craters in Volcanello, Lipari Islands 75 215. Section of the strata at the edge of the volcanic vent on the east side of Elie Harbour 76 216. Agglomerate of neck on shore at Ardross, two miles east from Elie 77 217. Ground-plan of volcanic neck, Elie Harbour, showing circular disposition of the stratification 80 218. Section across the great vent of Kincraig, Elie, on a true scale, vertical and horizontal, of six inches to a mile 81 219. Dyke in volcanic neck, on the beach, St. Monans 82 220. Section of part of crater rim, Island of Volcano 83 221. Dyke rising through the agglomerate of a volcanic vent; Kincraig, Elie 84 222. Radiating columnar dyke in the tuff of a volcanic vent. Rock and Spindle, two and a half miles east from St. Andrews 86 223. View of part of the shore front of the great vent at Kincraig, looking westward, with the columnar basalt in front 88 224. Plan of volcanic neck on beach near St. Monans 89 225. Columnar basalt in the neck of Kincraig, Elie, seen from the west 90 226. Section across Largo Law 91 227. Vein of "white-trap" cutting black carbonaceous shales, a little west from St. Monans Church 92 228. Section at Belvedere, S. W. of Exeter 97 229. Diagram to show the unconformability and overlap of the Permian rocks in the Crediton Valley 97 230. Section of the volcanic series at Kellerton, Devonshire 98 231. Section of agglomerate overlain with sandstone and andesite, Posbury, Crediton 99 232. Diagrammatic section across Titterstone Clee Hill 102 233. Dyke on the south-east coast of the Island of Mull 119 234. Fissure left by the weathering out of a dyke 120 235. Plan of basalt-veins with selvages of black basalt-glass, east side of Beinn Tighe, Isle of Eigg 126 236. Arrangement of lines of amygdales in a dyke, Strathmore, Skye 130 237. Systems of joints in the dykes 132 238. Section of cylindrical vein or dyke, cutting the bedded lavas, east side of Fuglö, Faroe Islands 133 239. Joint-structures in the central vitreous portion of the Eskdale Dyke (B. N. Peach) 133 240. Microscopic structure of the vitreous part of the Eskdale Dyke 136 241. Section along the line of the Cleveland Dyke at Cliff Ridge, Guisbrough (G. Barrow), scale, 12 inches to 1 mile 147 242. Section along the course of the Cleveland Dyke, at the head of Lonsdale, Yorkshire (G. Barrow, in the _Memoirs of the Geol. Survey_, Geology of Cleveland, p. 61) 148 243. Section across the extreme upper limit of Cleveland Dyke, on the scale of 20 feet to one inch (Mr. G. Barrow) 149 244. Upper limit of Cleveland Dyke in quarry near Cockfield (after Mr. Teall) 149 245. Section along the course of the Cleveland Dyke across the Cross Fell escarpment (scale of one inch to one mile) 150 246. Branching portion of the great dyke near Hawick (length about one mile) 153 247. Branching dyke at foot of Glen Artney (length about four miles) 153 248. Basic veins traversing Secondary limestone and sandstone on the coast cliffs, Aidnamurchan 155 249. Section showing the connection of a Dyke with an Intrusive Sheet, Point of Suisnish, Skye 156 250. Section to show the connection of a Dyke with an Intrusive Sheet, Stirlingshire Coal-field 157 251. Intersection of dykes in bedded basalt, Calliach Point, Mull 158 252. Basalt veins traversing bedded dolerites, Kildonan, Eigg 159 253. Ground-plan of intersecting dykes in Lias limestone, Shore, Harrabol, East of Broadford, Skye 159 254. Compound dyke, Market Stance, Broadford, Skye 162 255. Section of coal rendered columnar by intrusive basalt, shore, Saltcoats, Ayrshire 164 256. Dolerite dyke with marginal bands of "white trap," in black shale, Lower Lias, Pabba 166 257. Map of the chief dykes between Lochs Ridden and Striven (C. T. Clough, Geological Survey, Sheet 29) 170 258. Basalt-veins traversing granophyre, St. Kilda 173 259. Section of scoriaceous and prismatic basalt, Camas Tharbernish, north shore of Canna Island 187 260. Banded amygdaloidal basalt showing layers of elongated and steeply inclined vesicles, Macleod's Maidens, Skye 191 261. Termination of basalt-beds, Carsaig, Mull 193 262. Breccia and blocks of mica-schist, quartzite, etc., lying between bedded basalts, Isle of Mull 197 263. Section of Knocklayd, an outlier of the Antrim basalt-plateau lying on Chalk 202 264. Diagram-Section of the Antrim Plateau 203 265. View Of Basalt escarpment, Giant's Causeway, with the Amphitheatre and Chimneys. (From a photograph by Mr. R. Welch) 207 266. Basalt-capping on the top of Ben Iadain, Morven 209 266_a_. View of the south side of Staffa, showing the bedded and columnar structure of the basalt 210 267. View of Rum from the harbour of Canna 216 268. Section of the cliffs below Compass Hill, Isle of Canna 218 269. Lava cutting out conglomerate and shale. Shore below Canna House 224 270. Section of shales and tuffs, with a coniferous stump lying between two basalt-sheets, Cùl nam Marbh, Canna 225 271. Dùn Mòr, Sanday. (From a photograph by Miss Thom) 226 272. View of the Dùn Beag, Sanday, seen from the south. (From a Photograph by Miss Thom) 230 273. View of Dùn Beag, Sanday, from the north. The island of Rum in the distance. (From a Photograph by Miss Thom) 231 274. Section of eastern front of Dùn Beag 232 275. Enlarged section on the western side of Dùn Beag 233 276. Geological map of the Island of Eigg 235 277. Section of the geological structure of the Island of Eigg 236 278. View of the Scuir of Eigg from the east 237 279. Natural section at the cliff of Bideann Boidheach, north-west end of the Scuir of Eigg 239 280. View of the Scuir of Eigg from the south 242 281. View of the Scuir of Eigg from the south-west of the Loch a Bhealaich, showing the bedded character of the mass 243 282. Section at the base of the Scuir of Eigg (east end) 244 283. Terraced hills of basalt plateau (Macleod's Tables), Skye 250 284. "Macleod's Maidens" and part of basalt cliffs of Skye 251 285. Intercalated group of strata between Basalts, An Ceannaich, western side of Skye 252 286. Escarpment of Plateau-basalts, Cliffs of Talisker, Skye 253 287. Section of the largest of Macleod's Maidens 254 288. Dying out of lava-beds, east side of Sandö, Faroe Isles 257 289. Lenticular lavas, western front of Hestö, Faroe Isles 257 290. Lenticular lavas, east side of Svinö, Faroe Isles 258 291. Section at Frodbonyp, Suderö, Faroe 258 292. Fissure (gjá) in a lava-field, Iceland. (From a photograph by Dr. Tempest Anderson) 262 293. Cones on the great Laki fissure, Iceland. (From a photograph by Dr. Tempest Anderson) 263 293_a_. Plan of small craters along the line of great Laki fissure, Iceland. (After Mr. Helland, reduced) 264 294. Slemish, a volcanic neck or vent on the Antrim plateau, seen from the north 272 295. Section of volcanic vent at Carnmony Hill (E. Hull) 272 296. Section of the east side of Scawt Hill, near Glenarm 273 297. Section of Neck of basalt, Bendoo, Ballintoy 273 298. Volcanic Neck of dolerite near Cushendall 274 299. Section of Volcanic Neck at 'S Airde Beinne, near Tobermory, Mull 274 300. Interior of the Volcanic Neck of 'S Airde Beinne, near Tobermory, Mull 275 301. Diagram to show the probable relation of the Neck at Carrick-a-raide, Antrim, to an adjacent group of tuffs 277 302. Section of agglomerate Neck at Maclean's Nose, Ardnamurchan 279 303. Diagram to show the probable relations of the rocks on the southern flank of Beinn Dearg Bheag 282 304. Section of Volcanic Vent and connected lavas and tuffs, Scorr, Camas Garbh, Portree Bay, Skye 284 305. Section of the Volcanic Series at Ach na Hannait, south of Portree, Skye 288 306. View of part of a Volcanic Neck at the eastern end of the island of Canna. (From a photograph by Miss Thom) 289 307. Columnar Basalt invading agglomerate of Volcanic Vent, Coroghon Mòr, Isle of Canna. (Height above 20 feet) 291 308. Columnar Basalt invading Volcanic conglomerate, north side of Alman Islet, Canna 291 309. View of neck-like mass of breccia, Brochel, Raasay 292 310. View of Volcanic Neck piercing and overlain by the Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe Islands. (From a photograph by Colonel Evans) 294 311. Section of the same Neck as that shown in Fig. 310 295 312. Volcanic Neck close to that shown in Figs. 310 and 311 296 313. Section of wall of another Neck of agglomerate in the same group with those represented in Figs. 310, 311, and 312 296 314. View of "Segregation-Veins" in a dolerite sill, Portrush, Antrim 300 315. View of Fair Head, from the east, showing the main upper sill and a thinner sheet cropping out along the talus 301 316. View of Fair Head from the shore. (From a photograph by Mr. R. Welch) 302 317. Section at Farragandoo cliff, west end of Fair Head, showing the rapid splitting up and dying out of an Intrusive Sheet slope 304 318. View of the Trotternish Coast, showing the position of the band of Sills 305 319. Columnar Sill intrusive in Jurassic Strata east of Kilmartin, Trotternish, Skye 306 320. View of the northern precipice (500 feet high) of the largest of the Shiant Isles. (From a photograph by Colonel Evans) 308 321. Section of thin Intrusive Sheets and Veins in carbonaceous shales lying among the Plateau-basalts, cliffs north of Ach na Hannait, between Portree Bay and Loch Sligachan 311 322. Upper part of Sill, Moonen Bay, Waternish, Skye, showing the divergence of veins 313 323. Section of the base of the Basalt-plateau with sill and dykes, Sound of Soa, Skye 314 324. Section of Dolerite Sill cut by another sill, both being traversed by dykes, Rudh' an Iasgaich, western side of Sleat, Skye 316 325. Section to show Bedded and Intrusive Sheets, Eigg 318 326. Ground plan of Sills at Ben Hiant, Ardnamurchan. 321 327. Section of two Sills in schistose grits, west end of Beinn na h-Urchrach, Ardnamurchan 322 328. Sill traversing bedded Basalts, cliffs of Stromö, at entrance of Vaagöfjord 323 329. View of the same Sill seen from the channel opposite the island of Kolter 324 330. Granulitic and coarsely foliated Gabbro traversed by later veins of felspathic Gabbro, Druim an Eidhne, Cuillin Hills, Skye 331 331. Scuir na Gillean, Cuillin Hills, showing the characteristic craggy forms of the Gabbro. (From a photograph by Mr. Abraham, Keswick) 335 332. Section across Glen Brittle, to show the general relations of the Bedded Basalts and the Gabbros 336 333. View of the crest of the Cuillin Hills, showing the weathering of the Gabbro along its joints and of a compound basic dyke which rises through it. (From a photograph by Mr. Abraham, Keswick) 338 334. Section across the Coire Uaigneich, Skye 341 335. Banded and puckered gabbro, Druim an Eidhne, Glen Sligachan, Skye 342 336. Banded structure in the Gabbro, from the ridge of Druim an Eidhne, between Loch Coruisk and Glen Sligachan 343 337. Banded and doubly folded Gabbro, Druim an Eidhne, 10 feet broad 345 338. Sketch of banded structure in the Gabbros of the hills at the head of Loch Scavaig 347 339. Outline of the hills of the Island of Rum, sketched from near the Isle of Eigg 350 340. View of Allival, Rum, sketched from the base of the north-east side of the cone 352 341. Section of foliated Gabbros in the Tertiary volcanic series of Allival, Rum 353 342. Altered Plateau-Basalts invaded by Gabbro, and with a Dyke of prismatic Basalt cutting both rocks, north slope of Ben Buy, Mull 357 343. Theoretical representation of the structure of one of the Gabbro bosses of the Inner Hebrides 362 344. Section through the Puy de la Goutte and Puy de Chopine 374 345. View of the Huche Pointue and Huche Platte west of Le Pertuis 376 346. View of Glamich, 2537 feet, Glen Sligachan. (From a photograph by R. J. A. Berry, M.D., lent by the Scottish Mountaineering Club) 380 347. Section across the north slope of Beinn an Dubhaich, Skye 383 348. Section from Beinn Dearg to Beinn an Dubhaich, Skye 385 349. Section at north end of Beinn na Cro, Skye 388 350. Ground-plan of basic dyke in Cambrian limestones truncated by granophyre which encloses large blocks of the dyke, Torrin, Skye 393 351. Section on south side of Beinn an Dubhaich, Skye, showing the truncation of a basalt-dyke 394 352. View of the hills on the south side of the head of Loch na Keal, showing the junction of the Granophyre and the bedded basalts 396 353. Section on south side of Cruach Tòrr an Lochain, Mull 398 354. Section at head of Allt na Searmoin, Mull 398 355. Section on south side of Beinn Fhada, Mull 399 356. Section to south of Loch na Dàiridh, Mull 400 357. Section of junction of south side of Loch Ba' Granophyre boss, with the bedded basalts, Mull 401 358. Mass of dark gabbro about two feet in diameter traversed by pale veins of Granophyre, lying on north slope of Creag na h-Iolaire, Mull 402 359. Section at Creag na h-Iolaire, Glen More, Mull, showing basalts and gabbros resting on and pierced by Granophyre 402 360. Section on north side of Orval, Rum 404 361. Junction of Quartz-porphyry (Microgranite) and basic rocks, south-east side of Orval, Rum 404 362. Junction of Granophyre and gabbro, north side of St. Kilda 410 363. Veins of Granophyre traversing gabbro and splitting up into thin threads, north side of St. Kilda 411 364. Pale Granophyre injected into dark basalt, South Bay, St. Kilda 412 365. Veins of Granophyre traversing a fine-grained gabbro and scarcely entering a coarse-grained sheet, west side of Rueval, St. Kilda 413 366. View of sills and veins of pale Granophyre traversing sheets of gabbro, west side of St. Kilda. (From a photograph by Colonel Evans) 414 367. Section of the sea-cliff below Conacher, St. Kilda, showing basic dykes in Granophyre 417 368. Triple basic dyke, sea-cliff, east side of St. Kilda 417 369. Jointed structure of the Granite near the top of Goatfell, Arran. (From a photograph by Mr. W. Douglas, lent by the Scottish Mountaineering Club) 419 370. Intrusive Rhyolite in the lower basalt group of Antrim, Templepatrick 427 371. Section across the southern slope of Carnearny Hill, Antrim 427 372. Section across the Granophyre Sills at Loch a' Mhullaich, above Skulamus, Skye 433 373. Section to show the connection of a sill of Granophyre with its probable funnel of supply, Raasay 436 374. Granophyre sill resting on Lower Lias shales with a dyke of basalt passing laterally into a sill, Suisnish Point, Isle of Raasay 436 375. Weathered surface of spherulitic Granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size 438 376. Plan of portion of the ridge north of Druim an Eidhne, Glen Sligachan, Skye, showing three dykes issuing from a mass of Granophyre 439 377. Weathered surface of spherulitic Granophyre, from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size 440 378. Plan of pale Granophyric dyke, with spherulitic and flow-structure, cutting and enclosing dark gabbro, Druim an Eidhne 441 379. Dyke (six to ten feet broad) proceeding from a large body of Granophyre and traversing gabbro, from the same locality as Figs. 375 and 377 442 380. Section of intruded veins of various acid rocks above River Clachaig, Mull 443 381. Pitchstone vein traversing the bedded basalts, Rudh an Tangairt, Eigg 445 382. Reversed fault on the eastern side of Svinö, Faroe Isles 454 383. Reversed fault on the north-east headland of Sandö, Faroe Isle 454 MAPS V. Map of the Permian volcanic districts of Scotland _To face p._ 106 VI. Map of the Tertiary volcanic region of the West of Scotland _To face p._ 296 VII. Map of the Tertiary volcanic district of the North-East of Ireland _To face p._ 446 CHAPTER XXIX THE CARBONIFEROUS VOLCANOES OF ENGLAND The North of England: Dykes, The Great Whin Sill--The Derbyshire Toadstones--The Isle of Man--East Somerset--Devonshire 1. THE NORTH OF ENGLAND The volcanic intercalations which diversify the Lower Carboniferous formations of Southern Scotland extend but a short way across the English Border, and although, over the moors and hills of the north of Cumberland and Northumberland, the Carboniferous sandstones, limestones and shales are well exposed, they present no continuation of either the plateau or puy-eruptions which play so prominent a part in the geology of Roxburghshire and Dumfriesshire. This deficiency is all the more noticeable seeing that the Carboniferous system is exposed down to its very base, in the deep dales of the North of England. Had any truly interstratified volcanic material existed in the system there, it could hardly fail to have been detected. But while contemporaneous volcanic rocks are absent, the northern English counties contain many intrusive masses of dolerite, diabase, andesite or other eruptive rocks, which may be found traversing all the subdivisions of the Carboniferous system. These eruptive materials have taken two forms: in some cases they rise as Dykes, in others they appear as Sills. Dykes.--With regard to the dykes, some are probably much later than the Carboniferous period, and consequently will be more appropriately considered in Chapters xxxiv. and xxxv. The great Cleveland dyke, for example, which runs across the Carboniferous, Permian, Triassic and Jurassic formations, is probably referable to the Older Tertiary volcanic period. One dyke known as the Hett Dyke, has been plausibly claimed as possibly of Carboniferous age. It runs in a W.S.W. direction from the Magnesian Limestone escarpment at Quarrington Hill, a few miles to the east of Durham, through the great Coal-field, across the Millstone Grit and Carboniferous Limestone, disappearing near Middleton in Teesdale. Its total length is thus about 23 miles. It varies in breadth from about 6 to about 15 feet, and appears to increase in dimensions as it goes westward.[1] [Footnote 1: Sedgwick, _Trans. Geol. Soc._ 2nd series, iii. part 1 (1826-28), p. 63; _Trans. Cambridge Phil. Soc._ ii. (1822), p. 21. Sir J. Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p. 543.] The age of this dyke cannot at present be satisfactorily fixed. It must be later than the Coal-measures through which it rises. Sedgwick long ago pointed out that though it reaches the escarpment of the Magnesian Limestone, it does not cut it; yet it is found in coal-mining to traverse the Coal-measures underlying the Limestone. He was accordingly inclined to believe it to be of older date than the Magnesian Limestone. At its western extremity it approaches close to the Great Whin Sill of Teesdale, though no absolute connection between the two has been established. Mr. Teall, however, has called attention to the similarity between the microscopic structure of the rock forming the Hett Dyke and that of the mass of the Whin Sill, and he is strongly inclined to regard them as belonging to the same period of intrusion.[2] [Footnote 2: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.] It is especially worthy of remark that in the course of its nearly rectilinear course across the Durham Coal-field, the Hett Dyke, where it crosses the Wear, is flanked on the north at a distance of a little more than two miles by a second parallel dyke of nearly identical composition. Between the two dykes, during mining operations, a sill about 20 feet thick has been met with, lying between two well-known coal-seams at a depth of about 60 fathoms from the surface, and extending over an area of at least 15 acres.[3] Microscopic examination of this sill by Mr. Teall proved that the rock presents the closest resemblance to that of the Hett Dyke.[4] In this case, it may be regarded as probable that the two dykes and the intermediate sill form one related series of intrusions, and the conjecture that the Hett Dyke may be connected with the Whin Sill thus receives corroboration. The age of the Whin Sill itself will be discussed a few pages further on. [Footnote 3: Sir Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p. 544.] [Footnote 4: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.] Of the other dykes which may possibly be coeval with the Hett Dyke we may specially note those which follow the same W.S.W. trend, for that strike differs from the general W.N.W. direction of most of the dykes. Two conspicuous examples of the south-westerly trend may be seen, one near Morpeth, the other north of Bellingham. The former dyke, as regards microscopic structure, is more nearly related to the majority of the series in the North of England. But that north of Bellingham (High Green) presents affinities both in structure and composition with the Hett Dyke,[5] and may perhaps belong to the same period of intrusion. [Footnote 5: Mr. Teall, _op. cit._ p. 244. _Quart. Journ. Geol. Soc._ xxxix. (1884), p. 656, and _Proc. Geol. Assoc._ (1886). See also Prof. Lebour, _Geology of Northumberland and Durham_, chap. xi.] The Great Whin Sill.--The geologist who, after making himself acquainted with the abundant sills among the Carboniferous rocks in the centre of Scotland, finds his way into Northumberland, meets there with geological features that have become familiar to him further north. The sea-cliffs of Bamborough and Dunstanborough, the rocky islets of Farne, the long lines of brown crag and green slope that strike inland through the Kyloe Hills and wind across the cultivated lowlands and the moorlands beyond, remind him at every turn of the scenery in the basin of the Forth. But not until he has traced these ridges for many miles southwards and found their component rocks to form there an almost continuous sheet does he realize that nothing of the kind among the Scottish Carboniferous rocks can be compared for extent to this display in the North of England.[6] [Footnote 6: The Whin Sill has been the subject of much discussion, and a good deal of geological literature has been devoted to its consideration. The writings of Trevelyan, Sedgwick, W. Hutton, Phillips and Tate are especially deserving of recognition. The intrusive character of the Sill, maintained by some of these writers, was finally established by the mapping of the Geological Survey, and was discussed and illustrated by Messrs. W. Topley and G. A. Lebour in a paper in the 33rd volume of the _Quart. Journ. Geol. Soc._ (1877), in which references to the earlier observers will be found. See also Prof. Lebour's _Outlines of the Geology of Northumberland_, 2nd edit. (1886), p. 92. The petrography of the Whin Sill is fully treated by Mr. Teall in _Quart. Journ. Geol. Soc._ xl. (1884), p. 640, where a bibliography of the subject is also given.] From the furthest skerries of the Farne Islands southwards to Burton Fell on the great Pennine escarpment, a distance in a straight line of about 80 miles, this intrusive sheet may be traced in the Carboniferous Limestone series (Map I.). There are intervals where its continuity cannot be actually followed at the surface, but that it really runs unbroken from one end to the other underground cannot be doubted by any one who has examined the region. This singular feature in the geology and scenery of the North of England is known locally as the Great Whin Sill.[7] From the rocky islets and castle-crowned crags of the coast-line it maintains its characteristic topography, structure and composition throughout its long course in the interior. So regularly parallel with the sedimentary strata does it appear to lie, that it was formerly regarded by many observers as a true lava-sheet, poured out upon the sea-floor over which the limestones and shales were laid down. But its really intrusive character has now been clearly demonstrated. Not a vestige of any tuff has been detected associated with it, nor does it ever present the usual characters of a true lava-stream.[8] Its internal structure and the wonderful uniformity in its character mark it out as a typical intrusive sheet. [Footnote 7: "Whin" is a common term in Scotland and the North of England for any hard kind of stone, especially such as can be used for making and mending roads. "Sill" denotes a flat course or bed of stone, and was evidently applied to this intrusive sheet from its persistent flat-bedded position and its prominence among the other gently inclined strata among which it lies. It is from this example in the North of England that the word "sill" has passed into geological literature.] [Footnote 8: On the coast at Bamborough and the Harkess Rocks the usual petrographical characters of the Whin Sill are exchanged for those of fine-grained amygdaloidal diabases arranged in distinct sheets, which in their upper parts are highly vesicular and show ropy surfaces--peculiarities suggestive of true lava-streams. But according to Professor Lebour the rocks are intrusive into limestone and shale (_Geology of Northumberland and Durham_, p. 98). Mr. Teall has expressed the suspicion that these rocks must have consolidated under conditions somewhat different from those which characterized the normal Whin Sill (_Quart. Journ. Geol. Soc._ xl. p. 643). They seem to be the only parts of the sill which present features that might possibly indicate superficial outflow.] Among the manifestations of the subterranean intrusion of igneous rocks in the British Isles the Great Whin Sill, next after the Dalradian sills of Scotland, is the most extensive. Its striking continuity for so great a distance, and the absence around it of any other trace of igneous action, save a few dykes, place it in marked contrast to the ordinary type of Carboniferous sills. The occasional gaps on its line of outcrop in the northern part of its course do not really affect our impression of the persistence of the sheet. They not improbably indicate merely that in its protrusion it had a wavy irregular limit, which in the progress of denudation has occasionally been not yet reached. For mile after mile the sill has been mapped by the Geological Survey in lines of crag across the moorlands, and as a conspicuous band among the limestones and shales that form the steep front of the Pennine escarpment, where it has long been known in the fine sections exposed among the gullies by which that noble rock-face has been furrowed. [Illustration: Fig. 176.--Section from the great Limestone escarpment on the west to the Millstone Grit hills east of Teesdale. 1. Silurian strata; 2. Carboniferous Limestone series; 3. The Great Whin Sill, which gradually rises to higher stratigraphical position as it goes westward; 4. Millstone Grit.] Along its main outcrop, the sill dips gently eastwards below the portion of the Carboniferous Limestone series which overlies it. But so slight are the inclinations, so gentle the undulations of the rocks in this part of the country, that far to the east of that outcrop the sill has been laid bare by the streams which in the larger dales have cut their way through the overlying cake of Carboniferous strata down to the Silurian platform on which they rest (Fig. 176). Among these inland revelations of the eastward continuation of the sill under Carboniferous Limestone strata, the most striking and best known are those which have been made by the River Tees, and of which the famous waterfalls of the High Force and Cauldron Snout are the most picturesque features. The distance of the remotest of these denuded outcrops or "inliers" from the main escarpment is not less than 20 miles. It is not possible to form an accurate estimate of the total underground area of the Whin Sill. In the southern half of the district, south of the line of the Roman Wall, where, the inclination of the strata being generally low, the same stratigraphical horizons are exposed by denudation far to the east of the main outcrops of the rocks, we know that the sill must have a subterranean extent of more than 400 square miles. Yet this is probably only a small part of the total area over which the molten material was injected. In the northern part of the district, the Carboniferous Limestone series is not exposed over so broad a stretch of country, and denudation has not there revealed the eastward extension of the sill. But there is no reason to suppose the sheet to be less continuous and massive there. We must remember also that the present escarpment has been produced by denudation, and that the intrusive sheet must have once extended westwards beyond its present limits at the surface. If, therefore, we were to state broadly that the Great Whin Sill has been intruded into the Carboniferous Limestone series over an area of 1000 square miles we should probably be still below the truth. The rock composing this vast intrusive sheet is a dolerite or diabase, which maintains throughout its wide extent a remarkable uniformity of petrographical characters. In this and other respects it illustrates the typical features of sills. Thus it is coarsest in texture where it is thickest, and somewhat finer in grain towards its upper and lower surfaces than in the centre. Among the coarser varieties the component crystals of augite are not infrequently an inch in length and occur in irregular patches.[9] Occasional amygdaloidal portions are observable, but these are not more marked than those to be found in the "whin-dykes" of the same region.[10] The amygdaloidal and vesicular fine-grained rocks of the Bamborough district may possibly be quite distinct from the main body of the Whin Sill. [Footnote 9: Sedgwick, _Cambridge Phil. Trans._ ii. p. 166. Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 643.] [Footnote 10: Messrs. Topley and Lebour, _Quart. Journ. Geol. Soc._ xxxiii. p. 418.] Under the microscope the rock is seen to consist essentially of the usual minerals--plagioclase, augite and titaniferous magnetic iron-ore. An ophitic intergrowth of the augite and felspar is observable, likewise a certain quantity of micropegmatite which plays the part of groundmass between the interstices of the lath-shaped felspars. Full details of the characteristics of the component minerals and their arrangement are given by Mr. Teall in the paper already cited. The main body of the sill is a sheet which sometimes diminishes to less than 20 feet in thickness and sometimes expands to 150 feet, but averages from 80 to 100 feet. It occasionally divides, as near Great Bavington, where it appears at the surface in two distinct beds separated by an intervening group of limestones and shales. Occasionally, as at Elf's Hill Quarry, it gives out branches which send strings into the adjacent limestone.[11] [Footnote 11: Messrs. Topley and Lebour, _op. cit._ p. 413.] Although in most natural sections it seems to lie quite parallel with the strata above and below, yet a number of examples of its actual intrusion have been observed. When traced across the country, it is found not to remain on a definite horizon, but to pass transgressively across considerable thicknesses of strata. Its variations in this respect are well shown in the accompanying table of comparative sections constructed by Messrs. Topley and Lebour.[12] It will be seen that while at Harlow Hill the sill is found overlying the Great Limestone of Alston Moor, at Rugley, five miles off it lies about 1000 feet lower down, far below the position of the Tyne-bottom Limestone. Still farther north, however, the sill west of Holy Island is said to lie 800 feet above the Great Limestone and to come among the higher beds of the Carboniferous Limestone series.[13] [Footnote 12: _Op. cit._ plate xviii.] [Footnote 13: _Op. cit._ p. 414.] The Whin Sill appears generally to thicken in an easterly or north-easterly direction. There are further indications that it was intruded from east to west. Thus, at Shepherd's Gap, on the Great Roman Wall, the dolerite, coming evidently from an easterly quarter, has broken up and thrust itself beneath a bed of limestone. Again, when the sill bifurcates the branches unite towards the east or north-east.[14] The sill can be proved to thin away to the west from Teesdale to the Pennine escarpment, and in Weardale the "Little Whin Sill" diminishes from 20 feet, till in three miles it disappears.[15] [Footnote 14: _Op. cit._ p. 415.] [Footnote 15: _Op. cit._ p. 419.] [Illustration: _Walker & Bontall sc._ Fig. 177.--Sections of the Carboniferous Limestone series of Northumberland showing the variations in the position of the Whin Sill. By Messrs. Topley and Lebour.] The strata in contact with the Whin Sill, both above and below, have been more or less altered. Sandstones have been least affected; shales have suffered most, passing into a kind of porcellanite, with development of garnet and other minerals.[16] Limestone often shows only slight traces of change, though here and there it has become crystalline. [Footnote 16: Mr. Teall, _op. cit._ xxxix. (1884), p. 642, and authors cited by him.] No trace of any boss or neck has been detected in the whole region which might be supposed to mark a funnel of ascent for the material of the Whin Sill. The Hett Dyke and the High Green Dyke, already noticed, may, however, have been possibly connected with the injection of this great intrusive sheet. No other visible mass of igneous rock in the region has been even plausibly conjectured to indicate a point or line of emission for the sill. It is certainly singular that in so wide a territory, where the whole succession of strata has been so admirably laid bare by denudation in thousands of natural sections, and where, moreover, much additional information has been obtained from lead-mining as to the nature of the rocks below ground, not a single vestige of tuff, agglomerate or interstratified lava has been up to the present time recorded, unless the Harkess rocks already alluded to can be so regarded. Judging, however, from the analogy of the other districts of igneous rocks in Britain, we can hardly resist the conclusion that the Great Whin Sill is essentially a manifestation of volcanic action, that it was connected with the uprise of basic lava in volcanic orifices, and that the subterranean energy may quite probably have succeeded in reaching the surface and ejecting there both lavas and tuffs. It appears to be certain that any vents which existed cannot have lain to the west of the present escarpment of the sill, for no trace of them can be found there piercing the Carboniferous or older formations. They must have lain somewhere to the east in the area now overspread with Millstone Grit and Coal-measures, or still farther east in the tract now concealed under the North Sea. The evidence of the sill itself, as we have seen, corroborates this view of the probable situation of the centre of disturbance. The question of the geological age of the sill is one of considerable difficulty, to which no confident answer can be given.[17] The injection of the diabase must obviously be considerably later than the highest strata through which it has risen; that is, it must be younger than some of the higher members of the Carboniferous Limestone series. But here our positive evidence fails. [Footnote 17: See Messrs. Topley and Lebour, _op. cit._ p. 418.] The Sill is traversed by the same faults which disrupt the surrounding Carboniferous rocks. It is therefore of older date than these dislocations. Its striking general parallelism with the shales and limestones probably proves that it was intruded before the rocks were much disturbed from their original horizontal position. But the manner in which the intrusive rock has been thrust into and has involved the shales and limestones seems to indicate that these strata had already become consolidated and lay under the pressure of a great thickness of superincumbent Carboniferous strata. In the absence of all certainty on the subject it seems most natural to place the Whin Sill provisionally among the Carboniferous volcanic series with which petrographically and structurally it has so much in common. In Scotland the puy-eruptions continued till the time of the Coal-measures. If, before the close of the Carboniferous period, volcanic vents were opened somewhere to the east of the coal-fields of Northumberland and Durham, they might be accompanied with basic sills injected into the Carboniferous Limestone series, which was then lying still approximately horizontal under a thickness of from 3500 to 5000 feet of Carboniferous sedimentary deposits. These still undiscovered volcanoes seem to have been endowed with even more energy than those of Central and Southern Scotland, at least nowhere else among the Carboniferous records of Britain is there such a colossal manifestation of subterranean intrusion as the Great Whin Sill. 2. THE DERBYSHIRE TOADSTONES In the absence of any certain evidence that the Whin Sill belongs to the Carboniferous period, we must advance southward into the very heart of England before any clear vestiges can be found of contemporaneous volcanic eruptions among the members of the Carboniferous system. After quitting the lavas and tuffs of Roxburghshire and their brief continuations across the English border, we do not again meet with any truly bedded volcanic rocks in that system until we reach the middle of Derbyshire. In this picturesque district, famous for its lead-mines and its mineral waters, a feebly developed but interesting group of intercalated lavas, locally called "toadstones," has long been known. There is thus a space of some 150 miles across which, though the formations are there so fully developed and so abundantly trenched by valleys from the top to the bottom of the system, no volcanic vents nor any trace of Carboniferous volcanic ejections has yet been found. On the other hand, after the district of the "toadstones" is passed, the Carboniferous rocks are again destitute of any volcanic intercalations across the centre and south-west of England and over Wales, until after a space of about 150 miles they reappear in Somerset. The volcanic group of Derbyshire thus stands out entirely isolated. Lying in the Carboniferous Limestone, where that formation is typically developed, it presents an admirable example of a thoroughly marine phase of volcanic action (Map I.). One of the most prominent features in the geology of the centre of England is the broad anticlinal fold which brings up the lower portion of the Carboniferous system to form the long ridge of the Pennine chain that runs from Yorkshire to the Midland plain, and separates the eastern from the western coal-fields. This fold widens southwards until not only the Millstone Grit and Yoredale rocks, but the underlying Mountain Limestone is laid bare. A broad limestone district is thus exposed in the very heart of the country, ranging as a green fertile undulating tableland, deeply cut by winding valleys, which expose admirable sections of the strata, but nowhere reach the base of the system. The total visible depth of the limestone series is computed to be about 1500 feet; the Yoredale shales and limestones may be 500 feet more; so that the calcareous formations in which the volcanic phenomena are exhibited reach a thickness of at least 2000 feet. It is not yet definitely known through what vertical extent of this thickness of sedimentary material the volcanic platforms extend, but where most fully developed they perhaps range through 1000 feet, lying chiefly in the Carboniferous Limestone, but apparently in at least one locality extending up into the lower division of the Yoredale group. The area within which they can be studied corresponds nearly with that in which the limestone forms the surface of the country, or a district measuring about 20 miles from north to south, with an extreme breadth of 10 miles in an east and west direction. A special historical interest belongs to the Derbyshire "toadstones."[18] They furnished Whitehurst with material for his speculations, and were believed by him to be as truly igneous rocks as the lava which flows from Hecla, Vesuvius or Etna. But he thought that they had been introduced among the strata and "did not overflow the surface of the earth, according to the usual operations of volcanoes."[19] [Footnote 18: This word has by some writers been supposed to be corrupted from _tod-stein_, dead-stone, in allusion to the dying out of the lead veins there; by others the name has been thought to be derived from the peculiar green speckled aspect of much of the rock, resembling the back of a toad.] [Footnote 19: _An Enquiry into the Original State and Formation of the Earth_, 1778, Appendix, pp. 149, _et seq._] His views were published as far back as 1778, three years after Hutton read the first outline of his theory of the earth and made known his observations regarding the igneous origin of whinstones.[20] The first detailed account of the Derbyshire eruptive rocks was that given by Fairey,[21] which has served as the basis of all subsequent descriptions. Conybeare, in particular, prepared a succinct narrative from Fairey's more diffuse statements, and thus placed clearly before geologists the nature and distribution of these volcanic intercalations.[22] Subsequently the district was mapped by De la Beche and the officers of the Geological Survey, and the areas occupied by the several outcrops of igneous rock could then be readily seen.[23] [Footnote 20: _Trans. Roy. Soc. Edin._ i. p. 275, _et seq._ Hutton specially mentions the toadstone of Derbyshire as one of the rocks produced by fusion, p. 277.] [Footnote 21: _General View of the Agriculture and Minerals of Derbyshire_ (1811).] [Footnote 22: _Outlines of the Geology of England and Wales_ (1822), p. 448.] [Footnote 23: See Sheets 71 N.W., 72 N.E., 81 N.E. and S.E. and 82 S.W. of the Geological Survey of England and Wales.] Though the "toadstones" were believed to form definite platforms among the limestone strata, and thus to be capable
{'title': 'The Ancient Volcanoes of Great Britain, Volume 2 (of 2) by Archibald Geikie', 'source': 'Project Gutenberg', 'lang': 'en'}