356 A. J. Julies-Broicne — Lower Carhoniferou^^ in Devon. 



compared with the Upper Devonian series of North Devon the 

 natural inference is that the southern area was one in which the 

 process of deposition of sediment went on slowly. 



This slowness of deposition is still more marked in the case of 

 the Lower Culm, for if Dr. W. Hind is right in correlating the 

 Posidonomya Beds and the Chert-beds with the Pendleside Series, 

 there remains very little indeed to represent the Carboniferous 

 Limestone Series ; merely some dark shales which may be 50 or 

 perhaps 100 feet thick. 



Now if we assume that the whole area of sea-floor on which these 

 deposits were successively formed was being more or less rapidly 

 raised as a consequence of volcanic disturbance, and that submarine 

 vents were opened from which lava and ashes were ejected, have we 

 not a sufficient explanation of the facts ? On a submarine area lying 

 at some distance from land and undergoing upheaval only a small 

 thickness of sediment could accumulate ; in the vicinity of active 

 volcanic vents animal life could not continually flourish on the 

 sea-floor, and consequently limestones would either be thin and 

 sporadic as in the Upper Devonian, or entirely absent as in the basal 

 beds of the Culm. 



In South Devon, therefore, it would seem that not only have we 

 to accept the fact that there is very little to represent the Lower 

 Carboniferous Series, but we are furnished with a good and sufficient 

 reason for a fact which would otherwise be very difficult to explain. 



Applying the lesson thus learnt in South Devon to the problem 

 which has so long exercised the minds of geologists in North Devon, 

 it is evident that we may have to accept as a fact the absence of any 

 commensurate equivalent of the Carboniferous Limestone Series ; 

 for even if it should turn out that some part of the Pilton Beds are 

 of Carboniferous age, yet it is not likely that any great thickness of 

 such beds can come in between those which contain a Devonian 

 fauna and the Fremington Beds. 



It may be said that there is no evidence of volcanic activity 

 in North Devon either during Upper Devonian or Lower Culm time. 

 So far as we know at present there is little evidence of it, but there 

 is intrusive felsite at Bittadon near the junction of the Morte Slates 

 and the Pickwell Down Beds, and similar rock was traversed in the 

 Button Wood cutting on the Barnstaple and Lynton Eailway.* 

 This felsite was described by Professor Bonney in 1871,* and 

 regarded by him as probably of late Devonian age, but it may 

 with equal probability be referred to the Lower Carboniferous, for, 

 as Professor Bonney pointed out, the only clue to its age is the fact 

 of its being cleaved, which proves the intrusion to be " prior to the 

 close of the Carboniferous Period." Another dyke of intrusive 

 igneous rock (apparently basalt) occurs in the Fremington Beds 

 near Fremington, and Mr. Hamling informs me that it is not far 

 from the passage to the Lower Culm ; a little further south is a boss 

 of grey felspathic rock, the relations of which are more obscure. 



1 Summary of Proc;. Geol. Survey for 1897. 



2 Geol. Mag., Dec. II, Vol. V, p. 207. 



