24i4f GEOLOGICAL DESCRIPTION OF THE EARTHS CRUST. 



the solid crust of the earth ; the existence of such masses 

 is a very recent discovery, for which science is indebted 

 to my distinguished friend and travelling companion 

 Ehrenberg. 



| If in this short but general review of the mineralogical 

 constituents of the crust of the earth, I do not place imme- 

 diately after the simple sedimentary rocks, those conglome- 

 rates and sandstones, which are also partially sedimentary 

 deposits, and which alternate with argillaceous schists and 

 chalk in the secondary and older formations, it is only 

 because these conglomerates and sandstones are not composed 

 solely of the debris of eruptive and sedimentary rocks, but 

 contain also the detritus of gneiss, mica slate, and other 

 metamorphic masses. The obscure process of metamorphism, 

 and the influence it exerts, should therefore form the third 

 class of fundamental rocks. 



I have already had occasion to remark that the endogenous 

 ; or erupted rocks (granite, porphyry, and melaphyre), not 

 only act dynamically, shaking, elevating, inclining, and late- 

 rally displacing the superincumbent strata, but that they also 

 modify the chemical combinations of their elements, and the 

 nature of their internal structure : thus forming new kinds 

 of rocks, of which the gneiss, mica slate, and granular or 

 saccharoidal limestone (Carrara and Parian marble), may be 

 cited as examples. The schists of the silurian or devonian 

 periods, the belemnitic limestone of the Tarantaise, the dull 

 grey calcareous sandstone, containing fucoids of the northern 

 Apennines (inacigno\ often assume in their altered state a 

 new and brilliant appearance, which renders their recognition 

 difficult. The metamorphic theory has been established by 

 following step by step the successive phases of transforma- 



