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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 are points, of which the evidence must 
be sought in the works to which I refer. ‘The evidence adduced by 
Prof. Sedgwick is mainly to be found in the great fact of super- 
position, 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, amount- 
ing 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 surface of this 
island has of course been obtained by a careful attention to the po- 
sition 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 these 
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 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 Elie de Beaumont respecting the 
parallelism of contemporaneous elevations, whether true or false, 
could not fail to give an additional interest to geological researches, 
conducted on so large a scale as those of Prof. Sedgwick. Mr. 
Murchison’s mode of investigation may be described thus: that he 
has applied, for the first time, to the rocks below the Old Red Sand- 
