F. P. Mennell — The Northern Margin of Dartmoor. 413 



to be a synonym. If the genus is a synonym of Spiriferina as 

 Trechmann supposes, the specific name elongata should still be used, 

 unless it is preoccupied in that genus. Trechmann's explanation of 

 the comb-tooth structure by partial weathering before fossilization 

 does not appear convincing. 



Psioidea stands on a different footing from Clavigera and 

 Rastelligera, in that it was still left by me a genus ccelebs, the only 

 species figured being labelled Psioidea sp. innom. It seems desirable 

 to remedy this omission as, although this group is included in 

 Spiriferina by Trechmann, the latter genus seems unwieldy and must 

 be broken up. I therefore propose Psioidea gen. nov. with genotype 

 Spiriferina suessi, Zugmayer, var. australis, Trechmann, this course 

 having the advantage of making the invalid Psioidea, Hector, a 

 synonym of the valid Psioidea, Thomson, 1919, since Trechmann states 

 that the figure published by Hector under the name of Psioidea sp. 

 appears to represent his sub-species. Apparently some European 

 Rhastic forms, viz. the second group of Zugmayer's Dimidiatae, may 

 be transferred to Psioidea. 



M 



V. — Observations on the Northern Maegin of Dartmoor. 



By F. P. Mennell, F.G.S., M.I.M.M. 



ANY unsolved problems still remain in connexion with Devonshire 

 geology, and there is no more interesting area in the county than 

 that surrounding the Dartmoor granite mass. The investigation of this 

 part of Devonshire has not been completed by the Geological Survey, 

 and most of the northern margin of the Moor remains undescribed. 1 

 During 1914, 1915, and 1916 the writer at intervals spent a good 

 deal of time in studying the metamorphosed sediments on the north side 

 of the Moor, and although he left England with his investigations 

 incomplete this brief account of the results obtained will, it is hoped, 

 be of service, if only as an indication of what is to be found. 



I. East Okehent Section. 



It will be convenient to take as a starting-point in the description 

 one of those lines of section which offer fairly complete exposures of 

 the rocks across the contact zone or the greater part of it. The best 

 of these is that afforded by the valley of the East Okement River, not 

 far from Okehampton. Above Fartherford railway viaduct this 

 stream flows over the whole width of the metamorphic aureole almost 

 directly at right angles to the strike. The total distance is just over 

 a mile and a quarter, and the succession may be tabulated as 

 follows : — 



10. spotted and hardened shales, with gritty bands. 

 9. chiastolite hornfels. 



1 The Lyd Valley has been dealt with by the writer in Q.J.G.S., vol. lxxi, 

 pp. 623-38. A few general observations are summarized in Q.J.G.S., 

 vol. lxxii, pp. 83-4, 1917. Mr. E. H. Worth has recently read a paper before 

 the Geological Society on the Meldon area, of which only an abstract has yet 

 appeared. 



