tachylite glass, however, often contains lozenge-shaped crystals of plagioclase feldspar and small prisms of augite and olivine, but all these minerals very frequently occur mainly as microlites or as skeletal growths with sharply-pointed corners or ramifying processes. Palagonite tuffs are found also among the older volcanic rocks. In Iceland a broad stretch of these rocks, described as "the palagonite formation," is said to cross the island from south-west to north-east. Some of these tuffs are fossiliferous; others are intercalated with glacial deposits. The lavas with which they occur are mostly olivine-basalts. Palagonite tuffs are found in Sicily, the Eifel, Hungary, Canary Islands, and other places. Lava flow sources A second mode of occurrence of tachylite is in the form of lava flows. Basaltic rocks often contain a small amount of glassy ground-mass, and in the limburgites this becomes more important and conspicuous, but vitreous types are far less common in these than in the acid lavas. Thachylite may form at the edge of sills or thin dikes of basalt or diabase that rapidly cooled. Such edges may be as little as a millimetre thick. It merges internally into crystalline basalt. In the Hawaiian Islands, however, the volcanoes have poured out vast floods of black basalt, containing feldspar, augite, olivine, and iron ores in a black glassy base. They are highly liquid when discharged, and the rapid cooling that ensues on their emergence to the air prevents crystallization taking place completely. Many of them are spongy or vesicular, and their upper surfaces are often exceedingly rough and jagged, while at other times they assume rounded wave-like forms on solidification. Great caves are found where the crust has solidified and the liquid interior has subsequently flowed away, and stalactites and stalagmites of black tachylite adorn the roofs and floors. On section these growths show usually a central cavity enclosed by walls of dark brown glass in which skeletons and microliths of augite, olivine and feldspar lie imbedded. From the crater of Mt. Kilauea, thin clouds of steam rise constantly, and as the bubbles of vapor are liberated from the molten rock they carry into the air with them thin fibers of basalt that solidify at once and assume the form of tachylite threads. Under the microscope they prove to be nearly completely glassy with small circular air vesicles sometimes drawn out to long tubes. Only in the Hawaiian Islands are glassy basaltic