| OCR Text |
Show 108 GEOLOGY OF THE HIGH PLATEAUS. III. PROPYLITE AND ANDESITE. Richthofen has made two distinct orders of these rocks, each of equal taxonomic value with the other great groups, e. g., trachyte and basalt. There is no question that a tolerably sharp definition can be drawn between them, and that they are as readily distinguished in most cases by the unaided eye as by the microscope. The microscopic characters have been analyzed and described most thoroughly by Zirkel. But though the distinctions are well-drawn, and once mastered can seldom be confounded, the question arises, are they of sufficiently radical importance to warrant their separation into groups of such high rank as the trachytes and basalts ? It seems to me that Ave cannot do so without a violation of those fundamental principles which have gradually become almost universal in fixing primary characters. On purely chemical grounds so wide a distinction seems untenable, because the chemical difference is very small, and often so indefinite that it cannot be formulated. On mineralogical grounds the distinction is essentially no greater. Both of them are characterized by the predominance of plagio-clase, with accessory hornblende or augite and sometimes free quartz. The real difference is found in the respective textures, and in slight though constant differences in the modes of occurrence of the accessory minerals, and in some of the minor characteristics of the feldspars. But these distinguishing characters are precisely the same in their general nature and equivalent in degree to distinctions which are used in the trachytes, rhyolites, and basalts for separating the sub-groups, and which in other rocks have never risen to higher taxonomic values. If we follow the same methods and valuations in these rocks which we adopt in the other groups, it seems to me that we can only assign them to the rank of subdivisions of one principal group. With regard to the augitic andesites, Richthofen has placed them in the same major group as the hornblendic andesites. Zirkel, on the other hand, has placed them among the basalts. In deciding which of these two authorities it is best to adopt, the following considerations may be presented. It is not obvious that they use the term in precisely the same scope, nor embrace within their respective meanings quite the same rocks. We have certain rocks containing plagioclase, with abundant though sub- |
