264 The Founders of Geology LECT. 



It had long been known that greywacke or Transition 

 rocks covered most of the counties of Devon and Cornwall. 

 Closer examination of that region had shown that a con- 

 siderable tract of greywacke, now known as Culm-measures, 

 contained abundant carbonaceous material and even 

 yielded fossil plants that appeared to be identical with 

 some of those in the Carboniferous system. It was at first 

 supposed by De la Beche that these plant-bearing rocks 

 lay below the rest of the greywacke of that part of the 

 country. Murchison, however, from the evidence of his 

 clear sections in the Silurian territory, felt convinced that 

 there must be some mistake in regard to the supposed 

 position of these rocks, for he had traversed all the upper 

 greywacke along the Welsh border, and had found it to 

 contain no land-plants at all, but to be full of marine 

 shells. He induced Sedgwick to join him in an expedition 

 into Devonshire. The two associates, in the course of the 

 year 1836, completely succeeded in proving that the Culm- 

 measures, or Carboniferous series, lay not below but above 

 the rest of the greywacke of the south-west of England. 

 But what was that greywacke and what relation did it bear 

 to the rocks which had been reduced to system in Wales ? 



The structure of the ground in the south-west of Eng- 

 land is by no means simple, and, indeed, is not completely 

 understood even now. The rocks have been much folded, 

 cleaved and crushed. But besides these subsequent changes, 

 they present a great contrast in their lithological characters 

 to the Old Eed Sandstone on the opposite side of the 

 Bristol Channel. Neither Sedgwick nor Murchison could 

 find any analogy between the Devonshire greywacke and 

 the red sandstones, conglomerates and marls which expand 



