490 



traces of the upper Silurian strata, nor of the old red sandstone, 

 nor even of the mountain limestone in its ordinary aspect. On the 

 contrary, the next group met with in ascending order, is a culmi- 

 ferous series, the base of which distinctly reposes upon the above- 

 mentioned ancient rocks. This culmiferous deposit, far from ap- 

 pearing as a mere band, or at detached points, occupies about one 

 third of the large county of Devon, and a considerable adjacent 

 p irt of Cornwall ; its southern boundary ranging from Exeter on 

 the east, by Launceston, to St. Gennis in Cornwall on the west ; its 

 northern frontier running by Barnstaple and South Moulton to near 

 Wellington in Somersetshire. These culmiferous beds are shown to 

 contain thick beds of limestone, entirely dissimilar in structure and 

 fossil contents from any limestones of the underlying "grauwacke," 

 in which they had previously been merged. The culm measures 

 consist of grit, sandstone, shale and limestone; and these rocks, it is 

 said, are never affected by a slaty cleavage like the lower Silurian 

 and Cambrian rocks on which they rest. From this character, as 

 well as from their prevailing mineralogical st'-ucture and imbedded 

 fossil plants, the authors regard the culmiferous formation of Devon 

 as perfectly identical in age with other coal-fields, and as more par- 

 ticularly analogous to the culm-bearing strata of Pembrokeshire ; 

 a part of which also once passed for "grauwacke," but Mr. Mur- 

 chison has recently shown that it belongs to the South Welsh coal- 

 field, which is known by all geologists to rest upon mountain lime- 

 stone. 



Thus referred to the age of our ordinary coal, these strata of North 

 Devon are further proved to lie in a great trough, their southern edges 

 being turned up against the granite of Dartmoor, where they acquire, in 

 contact with the granite, when traversed by elvan dykes, many 

 characters of the metamorphic rocks, or those commonly termed 

 primary. The phgenomena of interference and alteration at the 

 junction are such as to give a comparatively modern date for the 

 eruption of the Dartmoor granite, and to explain why so much dif- 

 ficulty and ambiguity has prevailed in determining the age of soma 

 of the altered culm beds. 



Among other points which this survey oi^ Professor Sedgwick and 

 Mr. Murchison has settled, so far as Devon is concerned, is one of 

 the highest theoretical interest, and on which for more than two 



