COMPOSITION OF THE ROCKS. . 187 



crystalline in their structure ; such as granite, sienite, gneiss, 

 greenstone, porphyry, basalt, and ancient and modern lava. 

 They constitute the largest portion of the crust of the globe. 

 They are destitute of animal or vegetable remains, and hence 

 are called non-fossiliferous rocks. The aqueous rocks in- 

 clude the various deposits of clay, sand, gravel, conglome- 

 rates, sandstones, slates, etc. They are composed, essen- 

 tially, of the ingredients''of the igneous recks. They contain, 

 with the exception of a few rocks in the lower part of the se- 

 ries, the remains of animals and plants, and are hence called 

 fossilifcrous rocks. 



Rocks are subdivided into several groups. The un- 

 stratified or non-fossiliferous rocks, may be divided chemically 

 into two. The highly crystalline varieties, usually called pri^ 

 mary, such as granite, gneiss, mica slate and porphyry, form 

 one class, and the trappcan rocks, such as greenstone, basalt 

 and trap, form the other class. The difference in their chem- 

 ical constitution is this; that the first class contain about 20 

 per cent, more of silex, and from- 3 to 7 per cent, tessof lime, 

 magnesia and iron than the second class; 



The fossiliferous rocks are very various in composition, al- 

 though they correspond more nearly to the trappean variety 

 *' in containing less silica and more lime, magnesia and alu- 

 mina." 



The following are some of the most abundant rocks, 

 composed of the simple minerals. 



1. Granite is composed by the mechanical union of quartz, 

 feldspar and mica. The quartz is in irregular masses, the 

 feldspar often crystallized, and the mica in thin shining scales. 

 Hornblende sometimes displaces mica and forms what has 

 been called sienite. 



2. Gneiss is similar in composition to granite, but appears 

 to be formed by the destruction and deposition of the granite 

 by water. 



3. 3Iica slate is formed by quartz and mica, the latter pre- 



