TUTTACEOUS BEDS OF EAST FOEK CASTON. 



245 



such IS its real nature, if confining his examination to a hand-specimen and 

 imaware of its mode of occurrence. It has, however, some pecuHarities 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 observa- 

  tion has given me the impression that they are almost always scantily sup- 

 plied with them. These minerals are chiefly mica, hornblende, and plagio- 

 clase. There is also an unusually large quantity of peroxide of iron in a dif- 

 fused 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 pro- 

 nounced volcanic appearance. All of the little altered tufas shov^ 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 (Zeim seem to be extremely 

 susceptible to metamorphism. This is true not only of fine tufjis, but of 

 conglomerates which have a pulverulent matrix. But what is most remark- 

 able 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- 



