OP VOLCANIC ROCKS. 19 



Mineral Composition. — Trachyte is only inferior to rhyolite in the number of its 

 varieties. Yet its chemical composition is as simple as that of the latter, and ranges 

 within as definite limits. The rock appears to contain, on an average, from 60 to 65 

 per cent, of silica, and is in this respect, as in others, next allied to rhyolite. The 

 difference between the rocks belonging to both orders, having its fundamental cause 

 in the chemical composition, manifests itself externally in certain differences of charac- 

 ter, among which may be mentioned : the absence of quartz among the essential 

 ingredients of trachytic rocks, which probably contain no free silica at all ; the usual 

 predominance in them of oligoclase over sanidin ; the larger proportion in which horn- 

 blende participates in their mineral composition, and the fact of their specific gravity 

 exceeding that of rhyolite. Like almost all volcanic rocks, trachyte consists of a paste 

 in which are imbedded various crystallized minerals. This paste is of various colors, 

 and has usually a more or less vesicular texture, which, by its property of imparting 

 to the rock a certain roughness of touch, gave origin to the name. Chief in order 

 among the enclosed minerals are sanidin, oligoclase, mica, and hornblende. They vary 

 considerably as regards their relative proportion, and therefrom arises a number of 

 varieties which have been partly distinguished by separate names. More numerous 

 varieties, however, are occasioned by the differences of texture. Pumice-stone is of 

 frequent occurrence among the latter, but it never exhibits that perfect long-fibrous 

 and silken nature peculiar to it when being a variety of rhyolite. Obsidian, in differ- 

 ent grades of perfection, is no unusual modification of the trachytic masses, and it is 

 often filled with sphserolites, while no pearlite has yet been found having the chemical 

 composition of trachyte. Foliated structure may frequently be met with ; but the 

 folia are not of that exquisite fineness which is peculiar to those of rhyolite. The 

 modes of texture which alternate in the folia, are chiefly obsidian, pumice-stone, and 

 microcrystalline varieties. 



Subdivisions. — It appears that two natural groups of trachytic rocks may be 

 distinguished, which differ at the same time, from a mineralogical and chemical point 

 of view, and have therefore been arrived at similarly by the application of artificial 

 principles. 8 We distinguish with B. v. Cotta : 



8 I may here remark that I have endeavored to retain existing names, as much as possible, for designating the orders 

 and families distinguished in this present classification. What I have tried to establish as natural groups, may therefore, on 

 account of the similarity of nomenclature, coincide apparently, in many cases, with the groups established by artificial 

 principles, and named in accordance with them. The former do indeed coincide in a few instances with the latter; but in 

 the majority of cases, the limits of the application of the names differ widely when established from the two points of view 

 mentioned. It would require too much space to go into detail on this point in regard to every name. I will, therefore, 

 confine myself to an illustration regarding the order of trachytic rocks. B. v. Cotta, (with several other authors) unites 

 under the name of oligoclase-trachyte all volcanic rocks consisting chiefly of hornblende and oligoclase, with the exclusion 

 of certain dark-colored rocks, to which he applies the name andesite. The former name, if used in the meaning of that 

 author, comprises, therefore, our order of propylite, together with all those rocks for which the term oligoclase-trachyte has 

 been here applied. It is evident, from the different geological positions occupied by propylite and trachyte, as well as from 

 the distinct petrographical character which either of them exhibits when occurring in large accumulations, how much their 

 union would be opposed to a correct representation of natural relations. It is obvious, besides, that the union is unprac- 

 tical. It would actually prevent the possibility of a clear and simple geological description of those countries in which 

 propylite and trachyte occur together with other volcanic rocks. The distinction between both is so great that it has 

 scarcely ever failed to be noticed by unbiased observers in those localities where both rocks occur. To unite them because 



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