108 GEOLOGY OF THE HIGH PLATEAUS. 



III. PROPYLITE AND ANDESITE. Richthofen has made two 

 distinct orders of these rocks, eacli 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 micro- 

 scope. 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 we can- 

 not 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 con- 

 stant differences in the modes of occurrence of the accessory minerals, and 

 in some of the minor characteristics of the feldspars. But these distinguish- 

 ing 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 prin- 

 cipal 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 pre- 

 sented. 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- 



