﻿I860.] 



FOKBES BOLIVIA AND PERU. 



2D 



crystals of quartz distributed in abundance throughout the solid and 

 compact rock-mass. In many instances, as, for example (as before 

 mentioned), in the northern part of the Desert of Atacama, the greater 

 part, if not the whole, of the quartz contained in the trachytie beds is 

 so crystallized, and may easily be detached from the matrix as small 

 six-sided prisms, terminated at both ends by pyramids, and beautifully 

 smooth and lustrous. This could not have occurred unless the 

 crystals had been formed whilst the rock was in a perfectly liquid 

 state, and before the other mineral constituents had commenced 

 solidifying. 



This seems to point out one great distinction between volcanic and 

 plutonic rocks. In the former case the quartz has been the first 

 mineral element to crystallize from the liquid lava, as might 

 naturally be expected from the much higher temperature requisite 

 for its fusion. In the plutonic rocks (granite), however, the reverse 

 is the case ; the quartz has been the last element to assume the solid 

 state and crystallize, and is not found in true crystals, except where 

 the occurrence of drusic cavities or cracks in the solidifying rock 

 have accidentally occurred — and even then we only find the one end 

 of the crystal terminated by planes, — whilst at the same time the 

 easily fusible felspar has invariably crystallized before it. It is 

 evident that in these rocks the quartz has remained fluid or viscid at 

 a temperature much below its point of fusion, as it occupies the 

 spaces or intervals of the network formed by the crystallization of 

 the other constituent minerals of the rock, which are infinitely more 

 fusible than the quartz itself. 



5. Dloritk Rocks. — In geological age, the next rocks which we 

 come to are the diorites, seen in Sections Nos. 1 & 2, and which may 

 be termed Post-oolitic, from their cutting through the strata here 

 representing the Upper Oolitic series. 



They are composed exclusively of a white felspar, together with a 

 more or less dark-green hornblende ; the rock itself is generally 

 coarsely crystallized, but occasionally becomes so fine-grained in 

 texture as to admit of its being termed a greenstone. 



This rock is the same as that which occurs in Chile, and which has 

 been described by Darwin in his ' Report on the Geology of South 

 America,' under the name of " Andesite," I have preferred the name 

 " Diorite" until chemical examination may prove it to be distinct ; 

 as in external appearances it cannot be distinguished from the ordi- 

 nary diorites of Europe and other parts of the world. 



Quartz is never found in this rock when normal ; but at one or 

 two places, as, for example, Cerro de las Esmeraldas and Comanche, 

 where this rock breaks through the red sandstone beds, the d : orito 

 near to the point of contact occasionally contains some little quartz 

 grains, which it evidently has absorbed from the rock through which 

 it has broken ; in such cases a specimen might be obtained which is 

 mineralogically, but not geologically, a syenite. 



The felspathic constituent is generally of a pure white colour, 

 and triclinic in crystallization ; but, as anorthite, albite, andesine, 

 labradorite, and oligoclase also pertain to the triclinic felspars, it 



