F. P. Mennell — The Northern Margin of Dartmoor. 415 



distance, and then resume a more pronounced dip to the north. 

 A remarkable overfold is seen above the point where the Moor Brook 

 flows into the Okement, close to the mouth of an old adit. Further 

 on we find ourselves on a conspicuously banded type of hornfels, the 

 bedding planes of which form gently sloping surfaces in the riverbed. 

 This sometimes contains much biotite or chlorite, while at other points 

 it is extremely like some varieties of " calc-flinta", though it does not 

 seem probable that any great thickness of it was appreciably 

 calcareous. The banded hornfels has the appearance in the field of 

 being the thickest member of the metamorphic series, but more than 

 one overfold can be detected on close inspection. Its thickness may 

 therefore be more apparent than real, a supposition which is borne 

 out to some extent by its attenuation in the Taw section, though it 

 is true it may there be replaced in part by other kinds of rock. The 

 striped rocks are followed by some rather nondescript types of 

 hornfels, though even these are sometimes brown and sometimes grey 

 in different beds, and are here grouped with the banded series. Then 

 comes the lower epidiorite, marked in the river by a waterfall. It is 

 probable that a certain thickness of banded hornfels intervenes 

 between the epidiorite and the lower chiastolite hornfels, but as 

 several specimens selected for slicing proved to be merely highly 

 hornfelsed varieties of the epidiorite this point requires further 

 investigation. The epidiorite, it may be noted, sometimes contains 

 small crystals of tourmaline as well as secondary biotite. The 

 lower chiastolite hornfels, which is much more crystalline in 

 appearance than the upper band, is well seen along the path on the 

 left bank of the river, but apparently nowhere else. Beyond it there 

 are only very fragmentary exposures. The tuff is not seen in situ, 

 and its presence has to be inferred from fragments which are none 

 too common anywhere, but are more abundant on the right bank of 

 the river. The lower limestone, too, can only be discovered after 

 careful search ; in fact, I only found the outcrop in the river-bed after 

 my attention had been drawn to the probability of its occurrence by 

 what was to be seen in other localities. The andalusite hornfels is 

 also poorly exposed, though some felspathized and silicified bands of 

 it are to be seen on the river bank. Both this and the overlying 

 limestone can, however, be studied a little to the east on Watchet Hill, 

 near Belstone, the former in artificial exposures and the latter in 

 a number of prominent outcrops. 



The question now arises as to how far the section above described 

 can be considered complete. There is nothing in the exposures 

 themselves to suggest that there is any non-sequence, but we are led 

 to that conclusion by the presence, less than half a mile to the east, 

 of a band of tuff. Large masses of this rock are extremely abundant 

 along the lane running from Belstone to Okehampton via Fartherford, 

 and its position probably corresponds with the top of the upper 

 chiastolite hornfels already referred to ; indeed, it may be in situ above 

 that rock on the top of the West Cleave, overlooking the Okement, 

 a point which I did not have time to clear up. This is on the 

 assumption that the conspicuous craggy outcrops of dark hornfels 

 represent the chiastolite hornfels at that locality. It is true that the 



