236 GEOLOGICAL DESCRIPTION OP THE EARTH*S CRUST. 



of form and superposition, into stratified and unstratified, 

 schistose and compact, normal and abnormal, we trace out 

 and study the phenomena of formation and transformation 

 which are still going on before our eyes, they may be distri- 

 buted into the four following classes, according to their 

 mode of origin : 



1. Erupted rocks, which have issued from the interior 

 of the earth, either by volcanic action in a state of fusion, 

 or by plutonic action in a more or less softened state. 



2. Sedimentary rocks, precipitated or deposited from 

 liquids in which their particles were held in solution 

 or suspended ; these form the greater part of the secondary 

 and tertiary groups. 



3. Transformed or metamorphic rocks, in whicn the 

 texture and mode of stratification have been altered, either 

 by the contact or proximity of an erupted plutonic or vol- 

 canic rock (endogenous rocks), 2 * 1 or, as is more frequently the 

 case, by the action of vapours and sublimations, ( 242 ) which 

 accompany the issue of certain masses in a state of igneous 

 liquefaction. 



4s. Conglomerates, coarse or fine-grained sandstones or 

 breccias, consisting of mechanically divided fragments of 

 the three preceding classes. 



The production of these four kinds of rocks, as still 

 going on before our eyes by the pouring forth of vol- 

 canic masses in streams of lava, by the influence of these 

 masses on 'rocks previously hardened, by mechanical se- 

 paration or chemical precipitation from liquids charged 

 with carbonic acid, and by the cementation of the detritus 

 of rocks of every kind ; may be regarded as presenting 



