CLASSIFICATION OF ERUPTIVE ROCKS— TRACHYTES. 105 



rhyolites, and the same difficulty is experienced in finding a suitable system 

 of subdivision. In attempting to divide them, Richthofen has given two 

 subdivisions, sanidin-trachyte and oligodase-trachyte. The admission of an 

 oligoclase-trachyte involves a dilemma. If (as appears from his language) 

 he contemplates a rock in which oligoclase is the dominant feldspar, it can- 

 not, according to ordinary conceptions and definitions, be a trachyte at all, 

 but rather an andesite. If it means that it is abundant, though subordinate 

 to orthoclase, then the same is true of by far the greater portion of the whole 

 trachytic group. Again, sanidin-trachyte also seems objectionable as a 

 characteristic name of a subdivision of the trachytes, since sanidin is the 

 predominant mineral of the entire trachytic group. 



And yet my own limited studies have led me to the conviction that 

 Richthofen, with his rare insight into the real nature of the subjects he has 

 investigated, has hit upon a valid distinction, which we may safely follow. 

 Among the older trachytic eruptions we find rocks into which plagioclase 

 largely enters; indeed, to such an extent that we are often doubtful whether 

 it may not preponderate over the sanidin, or at least be very nearly equal 

 to it. In these same rocks we also find an abundance of hornblende and mag- 

 netite, giving them the dark ii'on-gray aspect which is presented by many 

 andesites. These hornblendic trachytes, however, are usually coarser and 

 rougher in fracture than the andesites, and the hornblende crystals are 

 rarely found in such perfection and full development as in the andesites, 

 and macroscopic inspection will generally enable us to form a very good 

 opinion as to which of the two we are dealing with, though sometimes we 

 are deceived. It is evident that such trachytes are not far removed from 

 the andesites, both in chemical and mineral constitution, and they sometimes 

 blend with them. 



On the other hand, we encounter among the later trachytes a different 

 series of macroscopic characters. They are very deficient in hornblende, 

 and more often contain mica (biotite). They are usually light-colored, 

 pale-gray, or red, or light brown, and almost never dark gray. In texture 

 they vary widely, but in no case do they ever suggest any affinity to 

 andesite, but rather to rhyolite. Some of the varieties, indeed, approach 

 rliyolite so closely that we often have still greater difficulty in separat- 



