AUSTEN — EXTENSION OF THE COAL-MEASURES. 45 



ascends, until at last it forms a barrier betwixt two distinct marine 

 provinces. 



The existence and progressive growth of an old ridge in this position 

 does not rest on zoological considerations alone. On the advancing land 

 of the Cotentin, the older palaeozoic series is brought up along the coast 

 east and west from beneath the sandstones and conglomerates of the 

 Montague de Roule, &c. On our side it is preserved in the fibrous 

 chloritic slates of the Bolt and Prawle, and in the crystalline rocks of 

 the Eddy stone. 



Prof. Sedgwick, speaking of the disturbances which have affected the 

 South Cornish district*, expresses his opinion that a force has acted 

 on the side of St. Austle, and a contemporary force on the south of 

 the Dodman, and between these two forces the present surface has 

 been broken and placed in its present contorted position ; and again 

 (p. 19) he says "we have indications of an elevatory axis ranging 

 (nearly east and west) along the south coasts of Devonshire and 

 Cornwall." 



These two axes, however, were not contemporaneous ; the Channel 

 axis had a priority of origin, and was the line of resistance which 

 contributed to cause the contortions in the sedimentary beds to the 

 north of it, when the granitic intrusions of the west of England took 

 place. 



Another proof is to be derived from a new branch of Geological 

 inquiry. Cleavage-structure of great masses is very generally ad- 

 mitted to have resulted directly from the mechanical tension to which 

 they have been subjected in the process of elevation : the limits of 

 an area of elevation are therefore clearly indicated, — being those 

 within which the cleavage-planes have an arched arrangement. This 

 law, for which we are indebted to Mr. D. Sharpef, with the deduc- 

 tion from it, — that the lines of vertical cleavage are the partings of 

 contiguous systems, will henceforth mainly guide us in determining 

 the extent, succession, and relative ages of early ranges. Applying 

 these considerations to our western counties, it will be found that, 

 taken in its broadest part, Devonshire presents an arch extending 

 north and south from Bickington to the South Hams : then follows 

 a band of vertical cleavage-structure ; beyond this a second arc is 

 seen to take its origin, and is continued south as far as the fibrous 

 chloritic slates which form the coast-line of the Prawle ; all this area 

 is only a small section of an arch ; but, by the aid of the foregoing 

 law, it becomes as satisfactory a proof to the geologist that it was once 

 continued on, as the face of a single stone on a ruined bridge may be 

 to the antiquarian of the character of the structure of which it once 

 formed a part. From this mere abutment preserved in the projecting 

 headlands of South Devon, we may replace with certainty an elevated 

 area to the South ; for without it, what now remains could not have 

 been produced. 



It might further be shown that the span of the Channel arch was 



* Quart. Journ. Geol. Soc. vol. viii. p. 10. 

 t Phil. Trans. 1852. 



