24 RICIITHOFEN NATURAL SYSTEM 



as Sorby has made in reference to those minute differences of texture, which can only 

 be detected with the aid of the microscope, and H. Rose in regard to the modifications 

 of silica and their causes, aided by exact chemical analysis and experiments made 

 with the view of enquiring into the differences of origin of such eruptive rocks as 

 differ from each other in texture, will, if further prosecuted, reveal the true nature 

 and cause of the properties which distinguish the rocks of these three different 

 orders. 



A few of the more palpable differences may here be noticed. Propylite is 

 essentially of greenish color, and some of its varieties resemble diorite in composition 

 and texture; andesite is of blackish color and approaches basalt in aspect; trachyte is 

 of various colors and shades, among which green and black are rarest of occurrence, 

 and in regard to its external characters resembles rhyolite more than any other rock. 

 Oligoclase, in trachyte, is frequently of the vitreous variety, scarcely ever so in pro- 

 pylite and andesite. Hornblende is an essential ingredient in these two, not so in the 

 former ; it is of fibrous texture and green color in most varieties of propylite, black in 

 andesite and trachyte. Mica is seldom wanting in the latter, while it is not of com- 

 mon occurrence in propylite and andesite. Titanic iron enters largely into the compo- 

 sition of these, and is contained in smaller proportion in trachytic rocks. The latter 

 excel by having the greatest variety of texture, while propylite has among the 

 three the most perfect porphyritic texture, which has given rise to its frequent 

 popular designation " porphyry ;" this name has never been applied to andesite or 

 trachyte. 



The enumeration of all these trifling differences is, however, insufficient to 

 express the marked distinction which exists in the external characters of propylite, 

 trachyte and andesite. As a similar, and even more conspicuous, distinction manifests 

 itself in their geological relations, we have to consider the existe .ce of those three 

 natural orders as a fact founded on observations, although we may be utterly unable . 

 to explain, and even to express it in words. 



Subdivisions. The remarks made in regard to the mineral composition of pro- 

 pylite have shown that the range of its varieties may conveniently, and in harmony 

 with geological occurrence, be subdivided into three parts : 



Farn. 1st. Quartzose Propyltte or Dacite. This embraces rocks which, though 

 having in general a similar composition to those of the following family, contain be- 

 sides, rounded grains of quartz, sometimes in considerable proportion. They occur in 

 the western part of Transylvania, where their outbreaks succeeded those of rocks of 

 the second family, and preceded those of andesite. 9 Similar rocks have been observed 

 in Sinaloa (Mexico). 



Fam. 2<?. Hbrnblendic Propylite. Rocks composed chiefly of hornblende and 

 oligoclase, as described above. This family embraces vastly the majority of all pro- 

 pylitic rocks observed, among others those of Washoe. Breithaupt's " tirnacite " 

 is one of its varieties. 



. ; 



9 Fully described by G. Stachc, loc. cit. 

 (62) 



