LI I! I-MIVUI-.N CM W> U 61 



avc ciysulliaed together promiscuously without any 

 definite arrangement, so that the rock presents much the 

 same texture no matter in what d: :t may be 



broken; 1 in a remarkable series of rocks they a: 

 schistose or foliated, that is, disposed in more or less 

 distinctly parallel folia or laminae of crystalline minerals. 

 norphously aggregated Rocks of this 



;//<, or , Compound. 



(a) Simplt. Composed essentially of one mineral, 

 though now and then with accessory ingredients. The 

 rocks of this sub-group are almost entirely of aqueous 

 origin, that is, they have crystallised from solutions in 

 water. Crystalline limestone, dolomite, gypsum, and 

 rock-salt may be taken as illustrative examples. A few 

 os occur in this form, as in hornblende-rock, but 

 most of them belong rather to the foliated type, 



(ff) Compound, Composed of two or more minerals 



in an info. y of proportions. Most of the rocks 



which constiti; nportant series are what are 



usually called Igneous ; that is, they have crystallised out 



.ten solutions like modern lava. They almost in- 



variably consist of silicates of alumina, with magnesia, 



;xDtash, soda, and varying proportions of iron oxides, 



1 Closer scrutiny will sometime* show that the constituents of a 



rock are not so irregularly aggregated as might be supposed. In 



granite, for instance, the quartz and felspar have crystallised 



rtr longer axes parallel ; in granophyre these minerals are 



intimately ami regularly intergrown ; in tpheralilic rocks spherical 



grains or pea-like aggregates show an internal fibrous radiating 



i... ... *. ) VI* MM ItfM 



riurr, and Bonmnncs luu in row*. i>iny qg 



in an ttoconsolidated condition have acquired what U called 

 strufturt, that is, their constituent* have been drawn out in the 

 direction of movement of the still uncnagraled mass, see p. 172. 



