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Show TUFACEOUS BEDS OF EAST FOEK OAISTOK. 245 such is its real nature, if confining his examination to a hand-specimen and unaware of its mode of occurrence. It has, however, some peculiarities not common in lavas, though not sufficiently marked to justify their exclusion from that category. It is an acid rock, carrying as much silica as some rhyolites or extremely siliceous trachytes. Feldspar, chiefly monoclinic, is very abundant and in conspicuous, though not very large, crystals. The most notable peculiarity is the abundance of accessory minerals, which is not a common character in volcanic rocks so highly charged with silica. Although they are seldom destitute of accessory minerals, my own observation has given me the impression that they are almost always scantily supplied with them. These minerals are chiefly mica, hornblende, and plagio-clase. There is also an unusually large quantity of peroxide of iron in a diffused state, which has given the rock a strong reddish or pink color. It is excessively hard and compact, and one of the most difficult to fracture of any in the whole district. Its chemical composition allies it most nearly to rhy-olite, but in texture and in mineral constituents it does not conform so nearly to that group. The base, when examined microscopically, is similar to that which is seen in rocks with a well-marked porphyritic habit. None of these peculiarities would be alone sufficient to affect the conclusion that it is a volcanic rock. My doubts have arisen from other considerations. Both above and below it are thin beds composed of materials which more or less closely resemble it, some so nearly that no appreciable distinctions can be drawn, and these are surely sediments deposited and stratified where they lie and altered by metamorphic action, some more, some less. A transition can be traced, by selecting from the different layers, ranging from tufas which have been but little altered to the extremely hard rock of pronounced volcanic appearance. All of the little altered tufas show that they are composed of water-worn volcanic sands and gravel, and in some which are greatly altered the original pebbles are still visible. The strata which are composed of volcanic debris seem to be extremely susceptible to metamorphism. This is true not only of fine tufas, but of conglomerates which have a pulverulent matrix. But what is most remarkable is that the result of the alteration is not a wholly crystalline rock, like gneiss or diorite or hornblendic schist, but one consisting of an amor- |