replace a considerable area of the rock by metasomatic substitution. The opal is amorphous, the chalcedony finely crystalline and often arranged in spherulitic growths that yield an excellent black cross in polarized light. The microcrystalline ground-masses are those that can be resolved into their component minerals in thin slices by use of the microscope. They prove to consist essentially of quartz and feldspars, which are often in grains of quite irregular shape (microgranitic). In other cases these two minerals are in graphic intergrowth, often forming radiate growths of spherulites consisting of fibers of extreme tenuity; this type is known as granophyric. There is another group in which the matrix contains small rounded or shapeless patches of quartz in which many rectangular feldspars are embedded; this structure is called micropoikilitic, and though often primary is sometimes developed by secondary changes that involve the deposit of new quartz in the ground-mass. As a whole those quartz-porphyries that have microcrystalline ground-masses are rocks of intrusive origin. Elvan is a name given locally to the quartz-porphyries that occur as dikes in Cornwall. In many of them the matrix contains scales of colorless muscovite or minute needles of blue tourmaline. Fluorite and kaolin appear also in these rocks, and the whole of these minerals are due to pneumatolytic action by vapors permeating the porphyry after it had consolidated but probably before it had entirely cooled. Many ancient rhyolitic quartz-porphyries show on their weathered surfaces numerous globular projections. They may be several inches in diameter, and vary from this size down to a minute fraction of an inch. When struck with a hammer they may detach readily from the matrix as if their margins were defined by a fissure. If they are broken across their inner portions are often seen to be filled with secondary quartz, chalcedony or agate: some of them have a central cavity, often with deposits of quartz crystals; they also frequently exhibit a succession of rounded cracks or dark lines occupied by secondary products. Rocks having these structures are common in north Wales and Cumberland; they occur also in Jersey, the Vosges and Hungary. It has been proposed to call them pyromerides. Much discussion has taken place regarding the origin of these spheroids, but it is generally admitted that most of them were originally spherulites, and that they have suffered extensive changes through decomposition and silicification. Many of the older quartz-porphyries that