694 Professor Sedgwick and R. I. Murchison, Esq., on the 



with the calp of Ireland, or the lower limestone shales of England, the culm 

 series of Devon will, as a whole, be very nearly the equivalent of our en- 

 tire carboniferous series. Considering, however, the enormous thickness of 

 the whole culm-measures and the paucity of fossils in all the inferior beds 

 of its lower division, we continue to place its base in a doubtful position. This 

 is a measure of necessary caution, especially when we bear in mind the ex- 

 treme difficulty, in the northern parts of our island, of drawing any precise 

 line of demarcation between the base of the coal-measures and the old red 

 sandstone*. 



Before the sections of North Devon had been made out, the culm-measures 

 were naturally included in the vast deposit of graywacke, of which we knew 

 neither the beginning nor the end. But to merge them in that series now, 

 is to reject all the direct evidence we possess, and all the analogies supplied 

 by the corresponding parts of our island. Lastly, there seems now to be no 

 difference of opinion as to the superficial distribution of the culm-measures ; 

 for the boundary line indicated roughly by us in 1836, and more in detail in 

 our previous paper, agrees, as far as it goes, with that lately published by Mr. 

 De la Beche. 



2. Fossils of Petherw'm and Barnstaple. — The fossiliferous slates in the 

 neighbourhood of these two places rise immediately from beneath the base 

 of the culm-measures. Their position is therefore symmetrical, and, as be- 

 fore stated, they appear on the whole to form a mineral passage into the 

 overlying system. In this way, we establish a mineralogical connexion be- 

 tween the North Devon and Cornish sections. But the connexion is made 

 incomparably more secure by help of the fossils. Those we have obtained 

 from Barnstaple, Croyde Bay, Marwood, &c. (all of them places in the highest 

 division. No. 5, of our North Devon section,) certainly agree as a group 

 with those of Petherwin, and have many species in common, as will ap- 

 pear by our lists. It is enough for our present purpose to quote the follow- 

 ing; Spirifer attenuatus, S. bisulcatus, Orthis interlineata, Atrypa concen- 



* The difficulty we have pointed out does not arise from any doubt respecting a great principle of 

 classification, but merely involves the adjustment of a boundary line. One of the best illustrations 

 of this difficulty is seen on the east coast of Scotland, north of St. Abb's Head. The old red sand- 

 stone first appears as a conglomerate, resting transversely on the edges of the graywacke ; and 

 then passes so insensibly into the red carboniferous rocks, that, through a considerable extent of 

 coast, it is hardly possible to draw any well-defined line between the two formations. We may, 

 however, observe by the way, that there are several pretty extensive tracts coloured as old red 

 sandstone in Macculloch's Geological Map of Scotland, which we believe every geologist who was 

 guided by the English types would class with the lower division of the mountain limestone ; and 

 there are one or two places in Dumfries-shire, where we think he has confounded the new with the 

 old red sandstone. 



