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Ch. VIII.] 



THE OLDER AND NEWER ROCKS. 



143 



enter here into a detailed account of what I have elsewhere 



termed 



now demonstrable in some countries that fossiliferous forma- 

 tions, some of them older than the Cambrian strata of our 

 table p. 139, others of the age of the Silurian strata, as near 



-L # I I T /*~*\ "1*1* * "1 



Nor 



as around Carrara in Italy, and some even of tertiary date, as 



-m a • • 



mica 



in the Swiss Alps, have been converted into gi 



schist, or statuary marble. The transmutation has been 



effected by the influence of subterranean heat acting under 



pressui 



steam and 



other gases permeating the porous rocks, and giving rise to 

 various chemical decompositions and new combinations, the 

 whole of which action has been termed < plutonic,' as express- 

 in^ in one word all the modifying causes brought into play 



& . ^ • r* t i 



at 



exem 



t 



7 — # 



To this Plutonic action the fusion of granite 

 itself in the bowels of the earth, as well as the super-induce- 

 ment of the metamorphic texture into sedimentary strata, 

 may be attributed ; and in accordance with these views the 

 age of each metamorphic formation may be said to be twofold, 

 for we have first to consider the period when it originated, as 

 an aqueous deposit, in the form of mud, sand, marl, or lime- 

 stone ; secondly, the date at which it acquired a crystalline 

 texture. The same strata, therefore, may, according to this 

 view, be very ancient in reference to the time of their deposi- 



comparatrv 



metamorph 



No 



>/ 



crystalline rocks were produced more 



abundantly at remote periods. — Several modern writers, without 

 denying the truth of the Plutonic or metamorphic theory, 

 still contend that the crystalline and non-fossiliferous forma- 

 tions, whether stratified or unstratified, such as gneiss and 

 granite, are essentially ancient as a class of rocks. They 

 were generated, say 

 state of the 



most 



time 



has been always on the decrease, until it became very incon- 



* See Lyell's Elements or Manual of 

 Geology, ch. xxxv. 



f See ' Elements :' Eemarks on lry 

 drothermal action, p. 731. 



kV 



