Geological Society. 451 



hands ; and which Professor Sedgwick grappled with the more reso* 

 lutely, in proportion as others shranic away from the task perplexed 

 and wearied. I need not attempt any detailed view of his system : 

 his First Class of Primary Stratified Kocks occupies the Highlands 

 of Scotland and the Hebrides, and appears in Anglesea and Caernar- 

 vonshire ; the crystalline slates of Skiddaw Forest, and the Upper 

 Skiddaw slate series come next. Above these is his Second Class, 

 or Cambrian and Silurian System. The Cambrian is divided into 

 Lower and Upper Cambrian, of which the former includes all the 

 Welsh series under the Bala limestone ; the two great groups of 

 green roofing slate and porphyiy on the north and south sides of the 

 mineral axis of the Cumbrian mountains (of Avhich groups the position 

 had previously been misunderstood), and parts of Cornwall and South 

 Devon. The Upper Cambrian System contains a lai'ge part of the 

 Lammermuir chain ; a part of the Cumbrian hills, commencing with 

 the calcareous slates of Coniston and Windermere ; the system of the 

 Ber^yns and South Wales ; all the North Devon, and a part of the 

 South Devon and Cornish series. Ascending thus through a series 

 of formations distinguished and reduced to order by the indefatigable 

 exertions and wide views of Professor Sedgwick, we arrive at tiie 

 Silurian system ; and here we must seek our subdivisions from the 

 rich results of the labours of Mr. Murchison. These subdivisions 

 were published in the summer of 1833. Like the Cambrian, the Si- 

 lurian is divided into a Lower and an Upper System, the former in- 

 cluding the Llandeilo flags and the Caradoc sandstones ; the Upper 

 Silurian 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 superposed 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 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 emj)loy and to improve all 

 the best methods of geological investigation. Prof. Sedgwick's 

 classification of the oldest rocks which form the surface of this 



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