354 A. J. Jukes-Browne — Lower Carboniferous in Devon, 



measure trough is as feeble as it seems to be on the northern side, 

 and if we can find any clue to the explanation of this phenomenon 

 in the south, it may be applicable to the northern area. 



There is also this advantage, that the surveys and researches made 

 by Mr. Ussher have established the Devono-Carboniferous succession 

 in South Devon and North Cornveall, which was formerly such 

 a doubtful matter. It now appears that in this area there is a much 

 more concentrated type of Upper Devonian, consisting mainly of 

 slates with interbedded volcanic rocks, and that its total thickness 

 is much less than in North Devon, so that there is less doubt about 

 the delimitation of the Devonian and Carboniferous systems. It is 

 true that the actual passage from one formation to the other has not 

 yet been recognised in section, but the limits within which the 

 transition may lie are much narrower. 



On the other hand, Mr. Ussher's work has shown that the 

 succession of the Lower Culm Series in South Devon is similar to 

 that in North Devon, being as follows : — 



4. Dark shales and mudstones with occasional limestones 



(Posidonomya Beds). 

 3. Hard cherty shales and chert beds (Goddoii Hill Beds). 

 2. Hard dark mudstones. 

 1. Dark grey shales. 



The beds below the cherty group are well developed near Ashton 

 and Trusham on the north-east side of Dartmoor, and are probably 

 part of the normal succession along the northern and western 

 portions of the area, but are seldom seen in actual infra-position to 

 the chert beds. The beds are everywhere so flexured and faulted 

 that Mr. Ussher has never yet ventured to publish any estimate of 

 their thickness. It can, however, be easily gathered from his 

 published descriptions of them that the thickness is relatively small, 

 and in response to my earnest request for some approximate idea of 

 their probable thickness he has kindly informed me that he thinks 

 the Posidonomya Beds may attain 150 feet, that the chert beds and 

 the hard mudstones below may attain 150 feet, and that he has 

 never seen more than 25 feet of the basal dark shales, though it is 

 quite possible that they have a thickness of 50 feet or more. He 

 also tells me that although he has separated the ' hard mudstones ' 

 which underlie the chert beds, yet it is quite possible that they 

 contain Kadiolaria and belong to the Chert Bed group. 



Mr. Ussher also wishes me to say that the statement of possible 

 thickness is merely an expression of the probabilities of the case as 

 impressed upon him by experience in the field. That, however, is 

 all that is necessary for the object of this communication, and my 

 readers will understand that the figures are not to be taken as 

 measurements. 



Another point of special interest about this southern area is the 

 presence of contemporaneous volcanic rocks both in the Upper 

 Devonian and in the Lower Culm. In the former they are of 

 sporadic or local occurrence, but are specially prevalent to the north 



