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[ 267 ] 

 XXIV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from vol. xxxvii. p. 464.] 



November 20th, 1918.— Mr. G. W. Lamplugh, F.R.S., 

 President, in the Chair. 

 HE following communication was read : — 



The Geology of the Meldon Valleys, near Okehampton, on 

 the Northern Verge of Dartmoor.' By Richard Hansford Worth, 

 M.Inst.C.E., F.G.S. 



The area dealt with lies between the London & South- Western 

 main railway-line, from a point a little east of Meldon Viaduct to 

 near Sourton, and the ridge of Dartmoor occupied by Black Tor, 

 High Wilhays, Yes Tor, and West Mill Tor, — being the greater 

 part of the valley of the Redaven and a portion of the valley oi 

 the West Okement. 



The southern extreme of this area is occupied by the Dartmoor 

 Granite, north of which are shales, in which occurs a patch of 

 limestone, and these are intersected by numerous bands of igneous 

 rock. 



The shales as a whole, with but slight local deviations, strike 

 north-east and south-west and dip north-westwards, the mean 

 angle of dip being about 50°. 



The sedimentary rocks are divisible into : — 



(1) An alumino-arenaceous series, extending from the granite northwards 



for a breadth of somewhat over half a mile. 



(2) A calcareous series, abruptly but conformably succeeding the last. 



(3) A limestone, which occurs a short distance south of the railway. 



(4) Radiolarian cherts a little above and a little below the horizon of the 



limestone. 



(5) An aluminous bed north of the railway. 



Of these, (1) consists of impure grits, which, being well within 

 the aureole surrounding the granite, have developed secondary 

 mica, a little tourmaline, and small well-formed rutiles. In 

 some places, at contacts with granitoid veins, andalusite is also 

 found. (2) Consists mainly of porcellanites with beds of black 

 chert-like rock. The characteristic mineral of the porcellanites is 

 wollastonite, but at contacts with the Meldon Aplite garnet, 

 idocrase, scapolite, axinite, and lepidolite are also developed. (3) 

 Shows little sign of metamorphic action. (4) Are cherts of the 

 character already well known as occurring in the Lower Culm- 

 Measures, and described by the late Dr. G. J. Hinde & Mr. Howard 

 Fox. (5) Is a dark-grey rock, almost black, the characteristic 

 mineral of which is chiastolite. All these rocks succeed each other 

 conformably, and there is no evidence of folding or repetition. 



In the sedimentary series planes of weakness have developed, the 

 surface -traces of which are broadly coincident with the strike, but 

 which frequently lie counter to the dip. These planes have been 

 more or less successfullv invaded bv at least three series of igneous 



