W. A. E. Ussher — The Culm of Devonshire. 13 



aspect from the lower series, although splintery shales (of the St. 

 Davids Exeter type) appear to be interstratified with them in places. 



Upon the one-inch map it is perfectly impossible to work out the 

 structure except in a broadly general way ; there is no room on a 

 small scale map to indicate the numerous folds, and it is quite impso- 

 sible to ascertain the effect and magnitude of the faults. 



The great Culm Measure Synclinal is not a simple curve, the beds 

 troughed by it being crenulated, so to speak, into an infinity of small 

 folds, and in this crinkling process they have no doubt been forced 

 to accommodate themselves to an area so much smaller than that of 

 their pristine extent, that we may suppose overthrust faults very 

 likely to have occurred, and, coupled with natural impersistence of 

 lithological horizons, lenticular association of grit and shale masses 

 and structural peculiarities such as may frequently be found in the 

 Middle, and sometimes in the Lower Culm Measures; these con- 

 siderations tend not a little to obscure the relations of the beds. 



Lower Culm Measures. — On their northern outcrop the beds forming 

 this series occupy a very much nari'ower tract than that to the south. 

 Their northern outcrojD is about two to three miles in breadth ; their 

 southern outcrop varies very considerably, its breadth from Dartmoor 

 northward through Okehampton being about five miles, whilst on the 

 east of Dartmoor it is about 15 miles, and about the same on the west 

 of Dartmoor through Lidford ; but it must be remembered that this 

 estimate in the southern part of the area is necessarily based on the 

 old geological Map, and that both in northern and southern districts 

 the Lower Culm Measures may occur far beyond the limits assigned 

 to them, through repetitions by faults or anticlinal axes in the areas 

 occupied by the Middle Culm Measures. 



The Lower Culm Measures, owing to the occurrence of limestones 

 and of the remarkable buff and grey shales and cherty grits of the 

 Coddon Hill type in them, and to their connection with the Upper 

 Devonian beds, naturally excited more interest and attention than the 

 other members of the series, except the Anthracite beds of the Middle 

 Culm Measures of the north ; but I do not think, at least in the 

 northern area, that they can be subdivided in anything like persistent 

 groups. If, for instance, we take the faulted and much-flexured lime- 

 stone patches of West Leigh, ^ Hockworthy, Whipcoats, and Kitton 

 Barton, it is not difficult to determine their boundaries ; but to esta- 

 blish their connection in a continuous series is by no means easy. 

 Furthermore, when we follow their horizon westward, although we 

 can have no hesitation in regarding the limestones of Bampton and 

 of Ven near Barnstaple, as the same series, the entire absence of con- 

 tinuity for such considerable distances forbids the hypothesis of a 

 persistent calcareous horizon, which could only be proved by systems 

 of faults of which we could not hope to obtain sufficient proof. From 

 their local development, and very partial occurrence, both in the 

 northern and southern areas, I regard the limestones as lenticular 

 masses in the shales and grits. 



1 Vide Trans. Dev. Assoc, for 1878 and 1879, Fossils in Culm Measures Lime- 

 stone, etc., by Rev. "W. Downes. 



