264 MR. E. H. EASTALL ON THE [May I906, 



This upper dyke is very variable in character. In parts it is a 

 dark greenish-grey, fine-grained rock of somewhat porcelain-like 

 appearance, translucent on thin edges, and with a very splintery 

 fracture. This facies strongly recalls the eurite of Cader Idris, 

 described by Prof. G. A. J. Cole & the late Mr. A. V. Jennings. 1 

 It passes gradually into a slightly-coarser type, in. which banding 

 becomes very conspicuous, in alternate streaks of reddish and blue- 

 grey colours. The darker minerals show a strong tendency to 

 aggregation into rounded patches, and this often passes into very 

 well-marked spherulitic structures. This banding is obviously a 

 flow-structure, and both it and the spherulites are very well brought 

 out by differential weathering. These flow-bands are often very 

 highly contorted, as if the magma had been very viscous during 

 intrusion. This dyke shows a striking resemblance to the rhyolite- 

 dykes of Druim-an-Eidhne in Skye, described and figured by 

 Mr. Harker. 2 Here also the rhyolitic affinities are very marked, 

 though the dyke is visibly an apophysis of the granophyre. Near 

 the eastern end of the dolerite-mass this dyke is nearly 20 feet 

 thick, and its structure is strikingly columnar ; the upper and lower 

 layers are highly contorted, while the middle part is comparatively 

 free from banding : where the dyke is thinner, practically the whole 

 of it shows complex flow-banding. 



The microscopic structure of this rock was described by the late 

 E. E. Walker, 3 and its silica-percentage was found to be about 72, 

 so that it is a distinctly-acid rock [4768]. 



1 cannot agree with Walker's conclusion that ' the felsite was 

 probably first intruded, and the diabase came up later ' (loc. cit.). 

 The dolerite-intrusion is most clearly cut by the acid dyke, which 

 must be the later of the two. (See sketch-map, fig. 2, p. 262.) 



The lower dyke is very similar in character, but less variable ; it 

 is a fine-grained rock of ' f elsitic ' type, showing no very definite 

 characters except slight banding. It runs parallel to the lower 

 margin of the dolerite, but thins out before reaching the wall on the 

 west of Combe Beck. It can be traced for a long distance west- 

 wards across the steep slopes of High Stile, and approaches very 

 near to the upper dyke. Unfortunately the ground is obscured by 

 screes, and very inaccessible, so that its western prolongation is 

 hidden; but I believe that it joins the upper dyke before the latter 

 merges into the granophyre. 



The published 6-inch Geological Survey-map shows a long felsite- 

 dyke, running from the south-eastern extremity of the dolerite for a 

 mile or so in a south-easterly direction past Low Wax Knott, with an 

 interruption where it passes beneath the lag-plane. The western 

 end of this is presumably the remarkable mass of spherulitic or 



i Quart. Jo urn. Geol. Soc. vol. xlv (1889) p. 433. 



2 'The Tertiary Igneous Eocks of Skye' Mem. Geol. Surv. 1904, p. 283 

 & pis. xi-xii. 



3 Quart. Journ. Geol. Soc. vol. lx (1904) p. 84. 



