Occurrences of Tacliylyte. 321 



the descent is greatest on days of bright sunshine interrupted 

 frequently by passing clouds, and that rain slightly increases the 

 rate of descent. 



A description was given of a scree on Hindscarth, Cumberland, 

 in which the stones lie with their longer axes pointing down the 

 slope ; and it was pointed out that the movement of the stones in 

 the way described would cause the surface-stones to fall off those 

 on which they rested, and that others would be dislodged during 

 their descent. A numerical estimate was then made of the total 

 amount of movement produced on a scree of a certain size by 

 expansion and contraction of the surface-stones, and after alluding 

 to the relative efficiency of this and other agents upon various screes, 

 the author concluded by pointing out that in the case of the moon 

 this might be almost the only agent at work. 



3. " On some Additional Occurrences of Tachylyte." By Gren- 

 ville A. J. Cole, Esq., F.G.S. 



An intrusive sheet, some 8 feet thick, among the basalts of 

 Ardtun Head in Mull, has selvages of tachylyte. The specific 

 gravity of the glass is 2*83, and in other respects it resembles the 

 examples already described from the west of Scotland. But in thin 

 section, numerous fairly translucent spherulites are seen, which 

 accumulate towards the inner part of the selvage until the glassy 

 material between them disappears, and they become polygonal by 

 contact. The rays of these spherulites are often alternately grouped 

 in brown or greyer bundles, both series exhibiting striking pleo- 

 chroism; but the brown fibres appear darkest when their longer 

 axes are parallel to the shorter diagonal of the nicol's prism, 

 while the greyish and less fibrous areas are darkest in the reverse 

 position. The author believes that two distinct minerals are 

 present, as in the spherulites of the ordinary granophyric structure ; 

 the browner rays may be pyroxenic, but the crystalline substances 

 produced under these conditions of hurried consolidation may be 

 far different from those developed in the more central portions of 

 the mass. Spherulites with pleochroic rays are the normal type in 

 basic glasses, and some occur even in some acid examples. 



An analysis of the Ardtun spherulitic tachylyte shows it to 

 resemble that of Beal in Skye, having 53 per cent, of silica and 

 nearly 6 per cent, of alkalies. 



An occurrence of tachylyte at Kilmelfort, Argyll, was noted, and 

 a description given of an example of great beauty from the Quiraing 

 in Skye. The latter rock shows, in section, a light-brown translucent 

 glass, with abundant cumulites and small brown spherulites with 

 radial structures. 



Near Bryansford, County Down, in Ireland, a basalt dyke occurs, 

 the selvage of which must have originally resembled the tachylyte 

 of the O^uiraing. The alteration that this glass has undergone 

 guides one in the search for tachylytes (palagonite and so forth) 

 among the Palaeozoic rocks of the British Isles, and an instance was 



