part 2] GEOLOGY OF THE MELDON VALLEYS. 91> 



Mr. F. P. Mennell 1 writes of the aplite as a large irregular 

 intrusion. But long before this, Frank Kutley, in 1889, had 

 recognized the rock as occurring in an intrusive dyke or sheet,, 

 which he believed extended over a mile in length (3) ; and, in 

 1893, C. A. McMahon had spoken of it as a long straight dyke. 



The extreme points at which the main dyke has been found in 

 situ are : to the south-west LXXVI, S.W. 6, old ice-ponds, 

 Sourton, and to the north-east LXXVI, S.E. 17, the boundary 

 of the enclosed land near the Redaven. The distance between 

 these extremes is 3344 yards, or nearly 2 miles, and the dyke cer- 

 tainly extends beyond 17 north-eastwards. On the map (PI. VIII) 

 full black circles and a full line indicate actual in-siiu expo- 

 sures ; open circles and a dotted line show where the mapping has 

 been done from surface-stone ; the thinner dotted line, marked 

 1 approximate,' covers a length where the grasslands practically 

 obliterate all indications, and where it is known that for some 

 distance the dyke never reached the surface. 



LXXVI, S.E. 17, is part of the well-known Meldon exposure ; 

 near this occur many surface-boulders of the rock, and formerly a 

 larger number existed. 



The occurrence of aplite at LXXVI, S.W. 6, was ascertained 

 by me in 1903. The old ice-house had been sunk below ground- 

 level, and in the excavation the dyke had apparently been struck. 

 In May and June 1910, as a result of prospecting, the dyke, 

 4 feet wide, was found at LXXVI, S.W. 5, under 6 feet of 

 rubble-head. About 125 feet away to the south-west, at LXXVI, 

 S.W. 6, it had degenerated into three parallel veins in the shale, 

 from k to 1| inches thick, the two principal veins being 18 inches 

 apart. North-east from LXXVI, S.W. 5, in a length of 37 feet r 

 the dyke became reduced to 34 feet in width. A trench was 

 then dug at LXXVI, S.W. 7, about 2-50 feet from 0, and in 

 this and another pit only 67 feet from 5 no Meldon aplite was 

 found ; but in the line of the dyke the shale for a width of 6 feet- 

 was altered to a pale-grey porcellanite. Similar conditions pre- 

 vailed at the hedge round the Vellake enclosures. Obviously the 

 dyke exists below the present rock-surface, which it has not reached. 

 Within the Vellake enclosures no prospecting has taken place, 

 beyond a superficial examination, which has failed to detect any 

 boulders of aplite. 



On leaving the enclosures and entering South Down, we find 

 that the dyke is traceable by surface-indications, or by trial-pits, 

 quarries, or in -si tit exposures, for the whole of the remainder of its 

 ascertained length.- It will be obvious from the map that the dyke 

 does not pursue a straight course, nor are its deviations lixed by a 

 constant underlie, coupled with the varying level of the ground. 



Two offshoots, other than mere veins, have been detected — the 

 one at LXXVI, S.E. 19-04-65, the other at LXXVI, S.E. 23. 

 Both have been quarried. 



1 Q. J.G.S. vol. lxxi (1915-17) p. 636. 

 Q. J. G. S. No. 298. x 



