144 



DIFFERENCE IN TEXTURE OF 



[Ch. VIII. 



siderable in the Oolitic and Cretaceous periods, and quite 

 evanescent before the commencement of the tertiary epoch. 



Now the justness of these views depends almost entirely on 

 the question whether granite, gneiss, and other rocks of the 



same order ever 



originated 



at the surface, or whether 



according to the opinions above adopted, they are essentially 

 subterranean in their origin, and therefore entitled to the 

 appellation of hypogene. If they were formed superficially in 

 their present state, and as copiously in the modern as in the 

 more ancient periods, we ought to see a greater abundance of 

 tertiary and secondary than of primary granite and gneiss ; 

 but if we adopt the hypogene theory before explained, their 

 rapid diminution in volume among the visible rocks in the 

 earth's crust in proportion as we investigate the formations 

 of newer date, is quite intelligible. If a melted mass of 

 matter be now cooling very slowly at the depth of several 

 miles beneath the crater of an active volcano, it must remain 



■ 



invisible until great revolutions in the earth's crust have 

 been brought about. So also if stratified rocks are now by 

 hydrothermal action, or under the influence of intensely 

 heated steam and other gases undergoing semi-fusion and 

 econstruction far underground, it will probably require the 



l 



many 



surface and exposed to view, even at a single point. To 

 effect this purpose there may be need of as great a develop- 

 ment of subterranean movement as that which in the Alps, 

 Andes, and 



Himalay 



ammonites 



■b 



16,000 feet. By parity of reasoning we can hardly expect 

 that any tertiary rocks of the hypogene class will have been 

 brought within the reach of human observation, save at a few 

 isolated points, seeing that the emergence of such rocks 

 must always be so long posterior to the date of their origin, 

 and still less can formations of this class become generally 

 visible until so much time has elapsed as to confer on them 

 a high relative antiquity. Extensive denudation must also 

 combine with upheaval before they can be displayed at the 

 surface throughout wide areas. 



All geologists who reflect on subterranean movements now 





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