24 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



marked in general, in the earliest rocks, where the action 

 has been going on the longest. 



Composition op Igneous Rocks. 



Since an igneous rock is a mixture of minerals, we do 

 not expect to find a definite or unchanging composition 

 in any one variety. The minerals present depend on the 

 chemical composition of the molten magma from which 

 it cooled ; and within certain limits this composition may 

 have varied indefinitely. But this variation is governed 

 by certain rules. 



All igneous rocks contain a large proportion — from 

 thirty to seventy -five per cent — of silica. But those 

 especially rich in silica contain the largest amount of 

 potash and soda, and very little lime, magnesia, or iron ; 

 on the other hand, those poor in silica contain a large 

 amount of lime, magnesia and iron, almost no potash, and 

 not much soda. The former are called acid^ the latter 

 basic rocks. Alumina is present in considerable amount, 

 averaging ten to fifteen per cent, in all rocks except the 

 extremely basic ones, in which it is sometimes almost 

 wanting. 



Corresponding with the chemical change is a change 

 in minerals. The common minerals in igneous rocks are 

 almost all silica compounds. We have first gwaWz, which 

 is pure silica. Then the feldsjMvs, which are silicates of 

 alumina with some other basic oxides. Orthoclase feldspar 

 contains potash and a little soda. Plagioclasc feldspar 

 contains soda and lime in varying proportions. These 

 are the chief light-colored minerals. The dark minerals 

 are mica, liornblevde, avgite, hjpersthene and olivine. 

 Mica is a complex silicate, containing alumina, potash, 

 magnesia and iron. There are two species ; one, 



