Metamorphic Rocks. 129 



terwards so eloquently illustrated by his disciple and friend 

 Playfair, whom I am proud to call my first master in geo- 

 logy, " that in all the strata we discover proofs of the mate- 

 rials having existed as elements of bodies, which must have 

 been destroyed before the formation of those of which these 

 materials now actually make a part."* We learn from 

 Professor Sedgwick, that in the north of England there are 

 chloritic slates alternating with countless contemporaneous 

 ribs of porphyry, as well as with trappean conglomerates and 

 slaty beds, derived mechanically from malerials of igneous ori- 

 gin. M. Abich of Diirpat considers that certain dark-green 

 grains disseminated through the lowest beds of the Lower 

 Silurian " Pleta,',' or Orthoceratite limestone of Russia, are 

 the detritus of the ancient augitic rocks of the Finnish fron- 

 tier.! The least fragment of an organic body in the lowest 

 deposits, it is evident, must have been encased in silt or mud, 

 and that silt or mud must have been derived from pre-exist- 

 ing rocks, and most probably rocks exposed on land to the 

 destructive power of meteoric agents. We are told by Mr 

 Lyell, that the Potsdam sandstone, the lowest of the Silurian 

 strata of North America, at the Falls of Montmorency, near 

 Quebec, is remarkable for containing boulders of enormous 

 size — the largest he ever remembers to have seen, he says, 

 in any ancient stratified rock. He measured some of them, 

 which were 8 feet long. They consist of the same gneiss as 

 that on which the sandstone rests. He also observed in the 

 same sandstone, on the borders of Lake Champlain, ripple- 

 marks on the surface of its flags. 



Several of the works of geologists which have been pub- 

 lished during the last year, have supplied much additional 

 evidence of metamorphic action ; none more important, I 

 may say more conclusive, than is contained in the work of 

 Sir R. Murchison on Russia, and of Mr Lyell on America, 

 and in a very valuable memoir by M. Virlet. As far as my 

 limits will allow, I will bring forward some of that evidence. 



With limited exceptions, true granites are rarely found in 

 the higher portions of the Urals, but they are of frequent oc- 



* Illustrations of the iluttonian Theory, p. 5. t Murchison 's Russia, i.28. 



