﻿34 



PROF. S. H. REYNOLDS ON THE 



[vol. lxxii, 



III. Petro graphical Details. 

 (a) Goblin Combe. 



In hand-specimens the lavas are dark-greenish or black rocks, 

 with abundant amygdules of calcite and chlorite. 



The lavas from the eastern exposure were described in this Journal , 

 vol. lx (1904) p. 152, as ;amygdaloidal olivine-basalts, and 

 further examination, supplemented by chemical analyses, confirms 

 this view as to their character. The extinction-angles of the 

 felspar-laths appear to indicate a somewhat acid labradorite. 



In the western area of Goblin Combe fresher rocks were found 

 by digging trial-holes than had been previously obtained : for 

 instance, that at spot 81, which has not only the augite, but also 

 part of the olivine unweathered. In all essential characters the 

 rock from 81 is identical with that from 33. A good deal of 

 dark isotropic glass is present between the crystals. In the rock 

 from 81, while the augite and felspars are fresh, the olivine is to 

 a great extent serpentinized. In the rock from 6, the olivine is 

 replaced by a carbonate, the felspars are much weathered, and most 

 of the augite in the interstices between the felspars is serpentinized. 



A comparison of the two freshest lavas, from the eastern and 

 western exposures respectively, yields the following results : — - 



0. 33 : Goblin Combe. 0. 81 -. Goblin Combe. 

 Eastern exposure. Western exposure. 



Specific gravity 2-84 to 2-86 2-80 to 2-81 



Si0 2 49*12 per cent. 48*49 per cent. 



K,0 0*41 0*46 



Na 2 0 2*37 2*39 



Li 2 0 not found not found 



The chemical results recorded above fully confirm the opinion 

 reached from a microscopical examination that these rocks are 

 typical olivine-basalts. In the low percentage of alkalies, and 

 particularly of potash, they contrast with the other rocks of this 

 area. ' 



The tuffs were fully described {op. supra cit. p. 154), and there 

 are only two additional facts to record. One of these is the presence 

 of well-preserved foraminifera in some of the tuffs of the western 

 exposure. The other fact is the occasional occurrence of well- 

 rounded pebbles of considerable size in the tuff at the eastern end 

 of the Combe. These include a pebble of crinoidal limestone 

 measuring 2^xlf inches, and one of chert about an inch and a 

 half long. 



(b) Uphill. 



The Uphill ' trap ' is a much-weathered, highly r-amygdaloidal 

 rock with a reddish colour owing to the abundant iron-oxide. The 

 amygdules may consist of calcite, or of calcite and chlorite. The 

 specific gravity of the freshest and least vesicular example obtain- 

 able (50) is 2*60, and its silica percentage 43*17. In thin sections, 



