part 2] THE (iEOT-()(iV OF THE ME LI) ON VALLEYS. 1**1 



On the Redaven the dyke reaehes the surface at 830 feet above 



Ordnance datum. Its highest point on Sourton Tors is 12-50 feet 

 above the same datum, and there the dyke is very narrow. None 

 of the companion aplite-dykes crop out above the 1250-foot level. 



The relations of the dyke with the surrounding shales are 

 interesting-. The original bedding of the shales can easily be seen 

 where they have been altered by the aplite. They are built up of 

 thin layers of varying original composition : some layers are mere 

 laminae, some may have a thickness of 12 inches. The original 

 composition differing, the alteration-products vary also, and their 

 history subsequent to alteration has not been uniform. In one 

 large inclusion in the dyke, marked ' M ' on Section A hereafter 

 mentioned, there are. in a depth of about ok feet, seventeen distinct 

 layers, omitting count of thin lamina?. These layers, as the inclu- 

 sion lies in the dyke, dip north-westwards 80°, and this is not far 

 out of accord with the general dip of the shale at this point. 

 Many layers have decomposed to a purple-brown sandy mud 

 (which owes its colour to iron and manganese). In some of the 

 larger inclusions matters have proceeded even farther, and these 

 layers have wholly disappeared, leaving water-channels in their 

 place. No great flow of water ever proceeds from these channels. 



C. A. McMahon (4) wrote of the dyke ' as ' infolding large 

 slabs of the slates in its arms.' At the date of his paper there was 

 but a small quarry on the northern bank of the Redaven that he 

 could inspect. During the past nine years much more extensive 

 quarrying operations have been pursued continuous with this. 

 These have shown very many slabs of shale in the dyke (they 

 are much less frequent south of the Redaven). Sections A & R 

 (fig. 2, p. 100) show the quarry-face at successive stages of its 

 development ; both are cross-sections of the dyke from north-west 

 to south-east. Section A was measured when the face was about 

 100 yards north-east of the Redaven, and Section B represents 

 conditions some 80 yards farther north-eastwards. The shale 

 inclusions are indicated by parallel-line shading, the lines fol- 

 lowing the bedding; similar shading marks each wall of the dyke. 

 Meldon aplite is shown by a stippled ground. 



It will be seen that the south-eastern wall of the dyke dips 

 at an angle of between 58° and 58°; but on the north-west side the 

 angle is reduced to between 21" and 21°. 



In Section A this alteration is evidently due to the wedge-shape 

 of the inclusions, all of which are widest at the top, and sonic of 

 which die out before the quarry-floor is reached. At the floor- 

 Level the total width of the dyke is (52 feet, and shale-inclusions 

 occupy 6| feet, the net thickness of aplite being 55| feet. At 

 1L feet above floor-level, the distance from wall to wall is 75 feet, 

 and a width of 10 feet is occupied by slialc-inclnsions, leaving a 

 net width of aplite of 50 feet, or almost exactly the same as 

 at floor-level. Section R also shows the net width of aplite at 

 the quarry-floor as 55| feet, but 20 feet above that level the net 

 width is 69 feet: possibly one of the inclusions has been floated up 

 to levels now denuded. 



i 2 



