PART II. C1IA 



Age of Metamorphic Rocks 



granular limestone, talc-schist, talcose-gneiss,rmca^^usschist," 

 and other varieties. In regard to the age of this vast assemblage 

 of crystalline strata, we can merely affirm that some of the upper 

 portions are altered newer secondary deposits : but we cannot 

 avoid suspecting that the disappearance both of the older second- 

 ary and primary fossiliferous rocks may be owing to their hav- 

 ing been all converted in this region into crystalline schist. 



It is difficult to convey to those who have never visited the 

 Alps a just idea of the various proofs which concur to produce 

 this conviction. In the first place, there are certain points where 

 strata of the Oolite, Lias, and Chalk have been turned into gra- 

 nular marble, gneiss, and other metamorphic schists, near their 

 contact with granite. This fact shows undeniably that plutonic 

 causes continued to be in operation in the Alps down to a late 

 period, even after the deposition of some of the newer secondary 

 formations. Having established this point, we are the more 

 willing to believe that many inferior fossiliferous rocks, probably 

 exposed for longer periods to a similar action, may have become 

 metamorphic to a still greater extent. 



We also discover in parts of the Swiss Alps dense masses of 

 strata of the age of the Green-sand and Chalk, which have as- 

 sumed that semi-crystalline texture which Werner called transi- 

 tion, and which naturally led his followers, who attached great 

 importance to mineral characters taken alone, to class them as 

 transition formations, or as groups older than the lowest second- 

 ary rocks. (See p. 154.) Now, it is probable that these strata 

 have been affected, although in a less intense degree, by that 

 same plutonic action which has entirely altered and rendered 

 metamorphic so many of the subjacent formations ; for in the 

 Alps, this action has by no means been confined to the immediate 

 vicinity of granite. Granite, indeed, and other plutonic rocks 

 rarely make their appearance at the surface, notwithstanding the 

 deep ravines which lay open to view the internal structure of 

 these mountains. That they exist below at no great depth we 

 cannot doubt, and we have already seen (p. 127.) that at some 

 points, as in the Valorsine, near Mont Blanc, granite and grani- 

 tic veins are observable, piercing through talcose gneiss, which 

 passes insensibly upwards into secondary strata. 



It is certainly in the Alps of Switzerland and Savoy, more 

 than in any other district in Europe, that the geologist is prepared 

 to meet with the signs of an intense development of plutonic 

 action ; for here we find the most stupendous monuments of 

 mechanical violence, by which strata thousands of feet thick have 

 been bent, folded, and overturned. (See p. 74.) It is here 

 that marine secondary formations of a comparatively modern 



