on the Physical Structure of Devonshire. 315 



1. An ill-defined group near the granite, supposed to be meta- 

 morphic. 



2. A great complex slate group, with two subordinate calcareous 

 zones, in some places swelling out to a great thickness. The lower 

 calcareous mass (called the Ashburton bands) pass into Cornwall, 

 and range through the greater part of the county ; the upper are re- 

 presented in the most striking form by the Plymouth and Torbay 

 limestone. 



3. A coarse red arenaceous group, like the second group of the 

 preceding chapter, immediately surmounts the Plymouth and Torbay 

 limestones, and like them is of enormous thickness. 



4. A great schistose deposit, striking with the other rocks in the 

 southern region, nearly east and west. The prevailing dip is south, 

 and it is not much contorted, but at length it is reversed to the north, 

 being thrown off by an anomalous mass of chlorite and mica slate 

 which occupies the promontories of Start Point and Bolt Head. 



5. Mica and chlorite slate j — the relation of which to the other part 

 of the series is unknown. 



The authors then contrast the two regions above described. In 

 the southern, trap rocks appear occasionally j in the northern they 

 are wanting. The slaty cleavage so common in the northern is want- 

 ing in the southern region, though the rocks are in many places so 

 fissile as to make good roofing slate, but in such cases they exfoliate 

 parallel to the stratification. 



In comparing the two regions they endeavour, first, to identify the 

 calcareous group of Linton (No. 1 of chapter ii.) with the calcareous 

 bands (No. 2) of this chapter. Secondly, to identify the coarse red 

 group of North Devon (No. 2, chap, ii.) with No. 3 of South Devon. 

 Lastly, to identify the great slate group of South Devon (No. 4) with 

 Nos. 3 and 4 of North Devon. The absence of the calcareous band 

 of North Devon (No. 3) is not considered to throw much difficulty in 

 the way of this classification. By way of general conclusion, the au- 

 thors consider all the above groups of North and South Devon to be 

 netverth&n the rocks of Snowdon and central Cumberland (lowest part 

 of the Cambrian system), and older (with a very limited exception in 

 North Devon) than the Silurian system : they therefore place them 

 in the upper and middle parts of the Cambrian system, from the more 

 ordinary appearance of which they are chiefly distinguished by the 

 greater abundance of calcareous matter and fossils. 



The organic remains of the lower strata of South Devon are 

 indeed so very dissimilar from those of the Silurian system that they 

 cannot have been formed in that aera. These fossils will be de- 

 scribed and published*. 



Chap. IV. — Culmiferous series of the third region. 



The authors first describe many sections to prove that the Culm 

 Measures occupy a great trough, and dip away on both sides from 



* The Rev. — Hennah has placed his rich and valuable collection of Ply- 

 mouth fossils at the disposal of the authors. Mr. Austen, F.G.S., has also 

 contributed largely from the neighbourhood of Newton Bushel. Major 

 Harding, F.G.S., and the Rev. D. Williams, F.G.S., have been the most 

 zealous collectors in North Devon. 



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