Metamorphic Bocks, 125 



were once sandstones formed under the sea in the palaeozoic 

 period. In like manner, the sedimentary rocks on the northern 

 frontier of Russia, where they approach the great granitic 

 and trappean region that stretches southward from Russian 

 Lapland, become so changed, that the shales are converted 

 into Lydian stone, the limestones into marbles, and the sand- 

 stones into indurated and sometimes granular quartz. These 

 are not partial local effects, but characterise a long line of 

 country in a broad zone. The authors observe, that " the 

 thorough examination of this great band of Silurian rocks, 

 more or less metamorphic, which lies between the purely 

 crystalline or azoic rocks of the north, and the wholly un- 

 altered Devonian and Carboniferous deposits on the south, 

 well merit the special attention of the geologist, mineralogist, 

 and chemical philosopher ; for the scale on which these opera- 

 tions of change has been conducted is gigantic. Our present 

 acquaintance with the phenomena is, however, sufficient to 

 convince us, that here, as in other countries, the consolida- 

 tion, rupture, and alteration, of large portions of the earth's 

 crust have been effected by the agency and eruption of igne- 

 ous and gaseous matter." A limestone — ascertained, both 

 by lithological characters and fossiliferous proofs, to belong 

 to the Devonian age, in which copper veins occur at a point 

 where it is intersected in a complicated manner by green- 

 stone porphyry — is converted, for a space 350 fathoms long, 

 and 20 wide, into a crystalline rock, in some places becoming 

 a pure white crystalline saccharoid marble, and associated 

 with it is a garnet rock, loaded with very beautiful and large 

 crystals ; a case somewhat analogous to that observed by Pro- 

 fessor Henslow in Anglesea twenty-five years ago,* and to 

 that in the neighbourhood of Christiania described by Mi 

 Lyell.t On the east flank of the Urals, south of Ekaterin- 

 burg, there is a succession of low ridges parallel to the main 

 crest of the chain, composed of metamorphic rocks, some of 

 them so micaceous that they might pass, the authors say, for 

 primary mica-schist ; others resembling gneiss, which a few 



* Cambridge Philosophical Transactions, vol. i. 

 t Elements of Geolosfy, vol. ii. p. 403. 



