412 DESCRIPTIONS OF ROCKS. 



and since rocks of each kind often have a vast extent oyer 

 the earth's surface, the distinction as to whether a rock is 

 hornblendic or augitic is of prominent geological interest. 



n. CLASSES OF ROCKS. 



Rocks are of different classes, according to their texture 

 and origin. 



1. Fragmental. A large part of common rocks were 

 formed of sand, or pebbles and sand, and are only consoli- 

 dated sand-beds or gravel- beds ; and other related kinds are 

 more or less consolidated mud-beds or clay-beds. The mud- 

 beds of an estuary, or of the shallow seas off a coast, and the 

 stratified sand and gravel accumulations of sea-shores and 

 valley formations, are precisely the kind of material which 

 by consolidation have made the fragmental rocks, the most 

 abundant rocks of the earth's surface. Each pebble, grain 

 of sand, and constituent particle of the mud, was derived 

 from preexisting rocks, and is either an actual fragment from 

 those rocks, or else a fragment altered by more or less com- 

 plete decomposition. The rocks are hence called fragmen- 

 tal. The pebbles, and often the sands, have a worn surface, 

 and this fact, together with the structure of the beds, affords 

 evidence that they are fragmental. They are also the sedi- 

 mentary rocks of geology ; for the material was for the 

 most part carried and dropped by waters as sediment is 

 carried and dropped — the waters mainly of the ocean which 

 then covered the continents. 



2. Crystalline. Other rocks are crystalline. The grains 

 are angular instead of worn, and they crowd upon or pene- 

 trate one another because made in one process of crystal- 

 lization. They are generally angular over a fractured sur- 

 face because of the cleavage planes, like the grains of a 

 surface of broken iron. Granite, trap, white marble are ex- 

 amples of crystalline rocks. When such a rock is distinctly 

 granular there is little difficulty in deciding upon its being 

 a crystalline rock. If too fine-grained for a positive conclu- 

 sion with the aid of a pocket-lens, the doubt may usually 

 be removed by tracing it along to places where it is coarser ; 

 and if none such offers, by the preparation of thin slices for 

 microscopic examination. 



Crystalline rocks have received their crystalline texture 

 in different ways. 



