24 LYELL'S ELEMENTS OF GEOLOGY. 



Contemporaneous origin of Four Classes of Rocks. 



after the other without injury to the buildings above. In like 

 manner the materials of the lower part of the earth's crust may 

 pass from a solid to a fluid state, and may then again become 

 consolidated ; or sedimentary strata may assume a new and meta- 

 morphic texture, while the strata above continue unchanged, or 

 retain characters by which their claim to high antiquity may be 

 recognized. During such subterranean mutations, the earth- 

 quake may shatter and dislocate the incumbent crust, or the 

 ground may rise or sink slowly and insensibly throughout wide 

 areas ;* or there may be volcanic eruptions here and there ; but 

 the great mass may not undergo such an alteration as to be re- 

 generated and composed of new rocks. 



As all the crystalline rocks may, in some respects, be viewed 

 as belonging to one great family, whether they be stratified or 

 un stratified, it will often be convenient to speak of them by one 

 common name. But the use of the term primary would imply 

 a manifest contradiction, for reasons which the student will now 

 comprehend. Ic is indispensable, therefore, to find a new name, 

 one which must not be of chronological import, and must express, 

 on the one hand, some peculiarity equally attributable to granite 

 and gneiss (to the plutonic as well as the altered rocks,) and, 

 on the other, must have reference to characters in which those 

 rocks differ, both from the volcanic and from the unaltered sedi- 

 mentary strata. I have proposed in the Principles of Geology 

 the term " hypogene" for this purpose, derived from vrto, under, 

 and ywoptu,, to be born ; a word implying the theory that granite, 

 gneiss, and the other crystalline formations are alike nether- 

 formed rocks, or rocks which have not assumed their present 

 form and structure at the surface. It is true that all metamorphic 

 strata must have been deposited originally at the surface, or on 

 that part of the exterior of the globe which is covered by water ; 

 but, according to the views above set forth, they could never have 

 acquired their crystalline texture, unless they had been modified 

 by plutonic agency under pressure in the depths of the earth. 



From what has now been said, the reader will understand that 

 the four great classes of rocks may each be studied under two 

 distinct points of view : first, they may be studied simply as 

 mineral masses deriving their origin from particular causes, and 

 having a certain composition, form, and position in the earth's 

 crust, or other characters both positive and negative, such as the 

 presence or absence of organic remains. In the second place, 

 the rocks of each class may be viewed as a grand chronological 

 series of monuments, attesting a succession of events in the for- 

 mer history of the globe and its living inhabitants. 



* See chap. 5. 



