78 MR. R. H. WORTH OX THE [vol. lxXV, 



but both quarries are now abandoned. During the last eight 

 or nine years stone has been taken from the dyke of the Meldon 

 aplite east and west of the Redaven, and two small quarries in the 

 same rock were opened several years ago on the flank of South 

 Down, while prospecting work has been executed near the old Ice 

 House on Sourton Tors. There is a quarry in the shales by the 

 Yellake, and slight excavations have been made in connexion with 

 three mines, now abandoned. 



The opportunities in the field being thus described, it remains 

 to state that my collection of microscope-sections numbers over 

 270, and that, by the kindness of my friend Dr. E. H. Young of 

 Okehampton, I have been permitted to examine a further six dozen 

 slides. While no exhaustive treatment of the geology of this area 

 is as yet possible, the main features have probably been ascertained. 



The map (PL YIII) which accompanies this paper has its limita- 

 tions : the essentials of structure are (I believe) accurately plotted; 

 but complete mapping from surface-indications would be laborious 

 beyond my opportunities. The boundaries of the quarter-sheets 

 of the 6-inch Ordnance Survey are shown on the map, and their 

 numbers given, it being my practice to refer to all localities and 

 specimens by the sheet-number, followed by a numeral. 



II. General Structure of the Area. 



The mass of the Dartmoor granite lies to the south-east : its 

 northern boundary is accurately shown where it crosses the Redaven 

 and curves round West Mill Tor, and again where (after sweeping 

 round Shilstone Tor) it crosses the West Okement and flanks Black 

 Tor ; between these points, on the slopes of High Wilhays and 

 Yes Tor, no detailed survey has been made. 



On the north-west the granite is succeeded by a series of shales, 

 the bedding of which is well defined. For a width across the 

 strike of at least two-thirds of a mile the strike is fairly constant, 

 and may be taken as N. 51|° E., the dip is always north-westwards 

 and varies between 22° ancl 76°, with a mean of about 50°. This 

 strike is not many degrees out of parallel w T ith a line which, drawn 

 through the summits of Rough Tor, West Mill Tor. and Yes Tor, 

 would if produced pass but 1300 feet south of Black Tor. Near 

 the foot of the Redaven and at the Limestone Quarry on the 

 western bank of the West Okement there is a local deviation in 

 the strike, a deviation accomplished with some degree of regularity; 

 the dip, however, continues to be north-westward or thereabouts. 



But at the northern end of the Railway Quarry, for a short 

 space between normal strikes and dips, there are contortions which 

 bring some of the strata into a vertical position and some hori- 

 zontal, and even cause sharp folds. With this single exception 

 the shales would appear to be tilted against the granite, but 

 otherwise relatively undisturbed. 



Planes of weakness have been developed, the surface-traces of 

 which are broadly coincident with the strike, but they frequently 



