174 Mr Whewell's Address to the 



Cambrian system contains a large part of the Lammernmir cbain ; a part 

 of tlic Cumbrian HillSj commencing with the calcareous slates of Coniston 

 and Windermere ; the system of the Berwyns and South Wales ; all the 

 North Devon^ and a part of the South Devon and Cornish series. As- 

 cending thus through a series of formations distinguished and reduced to 

 order by the indefatigable exertions and wide views of Professor Sedg- 

 wick, we arrive at the Silurian system : and here we must seek our sub- 

 divisions from the rich results of the labours of Mr Murchison. These 

 subdivisions were published in the summer of 1838. Like the Cambrian, 

 the Silurian is divided into a Lower and an Upper system, the former in- 

 cluding the Llandeilo flags and the Caradoc sandstones; the Upper Silu- 

 rian rocks being the Wenlock shale and limestone, the Lower Ludlow, 

 the Aymestry limestone, and the Upper Ludlow, which finally conducts 

 us to the tilestones or bottom-beds of the old red sandstone. 



That these various series of Cambrian and Silurian rocks are really su- 

 perposed on one another ; that they are justly separated into these groups ; 

 and that the smaller groups are truly of a subordinate nature, divided by 

 lines less broad than those which bound the great series of formations ; — 

 these arc points, of which the evidence must be sought in the works to 

 which I refer. The evidence adduced by Professor Sedgwick is mainly 

 to be found in the great fact of superposition, supported by the circum- 

 stances of dip, strike, cleavage, mineral character, and all the great inci- 

 dents of mountain-masses. To proofs of this kind Mr Murchison is able 

 to add the testimonj' of organic fossils, of which a vast and most instruc- 

 tive collection is figured in his work. These fossils of the Silurian sys- 

 tem, 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 supi^orted by that geo- 

 logical evidence which properly belongs to the subject. 



" In detecting order and system among the monuments of the most ob- 

 scure 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 surface of this island has of course been obtained 

 by a careful attention 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 slatj' cleavage of 

 rocks and their stratification ; for the directions of these two planes, though 

 each wonderfully persistent over large tracts, never, except by accident, 

 coincide. 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 



