Geological Society : — Anniversary of 1839. 221 



which finally conducts us to the Tilestones or bottom beds of the 

 Old Red Sandstone. 



That these various series of Cambrian and Sihuian rocks are 

 really superposed on one another ; that they are justly separated 

 into these groups; and that the smaller groups are truly of a sub- 

 ordinate nature, divided by lines less broad than those which 

 bound the great series of formations ; — these are points, of which 

 the evidence must be sought in the works to which I refer. The 

 evidence produced by Prof Sedgwick is mainly to be found in 

 the great fact of superposition, supported by the circumstances of 

 dip, strike, cleavage, mineral character, and all the great incidents 

 of mountain masses. To proofs of this kind Mr. Murchison is 

 able to add the testimony of organic fossils, of which a vast and 

 most instructive collection is figured in his work. These fossils 

 of the Silurian system, amounting in all to about 350 species, are 

 essentially distinct from those of the Carboniferous System and 

 Old Red Sandstone. This being so, the establishment of these 

 great divisions is supported by that geological evidence which 

 properly belongs to the subject. 



In detecting order and system among the monuments of the 

 most obscure and remote periods of the earth's history ; it may 

 easily be supposed that it has been necessary to employ and to 

 improve all the best methods of geological investigation. Prof. 

 Sedgwick's classification of the oldest rocks which form the sur- 

 face of this island has of course been obtained by a careful atten- 

 tion to the position and superposition of the mineral masses, and 

 by tracing the geographical continuity of the strata, almost mile 

 by mile, from Cape Wrath to the Land's End. In this manner 

 he has connected the rocks of Scotland with those of Cumberland ; 

 these again with those of Wales ; and the Welsh series, though 

 more obscurely, with that of Devonshire and Cornwall. In this 

 survey he has constantly kept before his eyes a distinction, known 

 indeed before, but never before so carefully and systematically 

 employed, between the slaty cleavage of rocks and their stratifica- 

 tion ; for the directions of these two planes, though each wonder- 

 fully persistent over large tracts, never, except by accident, coin- 

 cide. He has taken for his main guide the direction of the strata, 

 or, as it is called, the strike of the beds ; and in such a course, the 

 theory of Elie de Beaumont respecting the parallelism of contem- 

 poraneous elevations, whether true or false, could not fail to give 



