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



mineral can seldom be detected in the field, but its presence, often in 

 curious skeleton crystals, is revealed by the microscope. Tuff at this 

 horizon is well exposed at the Meldon railway viaduct, where it has 

 been described by General MacMahon, 1 and fragments indicate its 

 presence all the way from the village of Sourton {not Sourton Tors) to 

 near Sticklepath, though it may be absent in places owing to thrusts, as 

 appears to be the case in the bed of the Okement. The lower and 

 more important bed of tuff (not recognized prior to the writer's 

 observation 2 except at Sourton Tors) is only traceable by fragments 

 in the Okement Valley, but is seen between the river and Belstone, 

 and is extremely well exposed in the cliffs overlooking the River Taw. 



Structure of the Rocks. 

 It is worthy of remark that we have here two bands of limestone, 

 two of tuff, two of chiastolite hornfels, and two of epidiorite. In 

 such a disturbed region it is necessary to consider whether this state 

 of affairs may not be attributable to a great overfold of Alpine 

 character. Assuming the axis of such a fold to be somewhere in the 

 banded hornfels series, where there are certainly clear indications 

 of overfolding, the positions of the epidiorite bands on either 

 side of it would fit in quite well with the supposition, and, leaving 

 the limestone out of account, the chiastolite hornfels and tuff would 

 also be in the right position. The limestone, however, which is of 

 considerable thickness on the north, is not recognizable on the south, 

 though it is only right to say that the same appears to be the case 

 with the upper limestone itself on the West Cleave, only a few 

 hundred yards from the river. There is, however, the other (lower) 

 limestone to be accounted for, which occurs in quite the wrong 

 position below the tuff, and also the fact that this is underlain by 

 andalusite hornfels, which can scarcely be a metamorphosed equivalent 

 of the beds above the upper tuff, as these are often of a sandy nature. 

 Moreover, this succession of tuff, limestone, and andalusite hornfels in 

 descending order is seen along the margin of the granite all the way 

 from near South Zeal to beyond Lake, with a regularity which 

 is strongly against such important disturbances having taken 

 place as would be required to account for the want of correspondence 

 north and south of the axis of an overfold. It may also be noted 

 that it would be necessary to assume that some representative of the 

 beds numbered 6 occurs between the hornfels and lower epidiorite. 

 There is no indication of this, and in the Taw section, which is clearer 

 at this point, there is certainly nothing of the kind. "What is very 

 suggestive that there is no such simple major overfold as has been 

 discussed, is the fact that so many signs of overfolding are to be seen, 

 and yet in no case do the plicatious result in distinct beds being 

 brought into juxtaposition. This is more in keeping with the idea 

 of minor puckerings of the strata than that of orogenic movements on 

 a grand scale. It may be concluded, therefore, that the apparent 

 order of succession is also the true one. 



^.J.G.S., vol. xlix, p. 380, 1893. 

 2 Q.J.G.S., lxxii, p. lxxxiv, 1917. 



