36 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



But in many examples in the south of Scotland, Argyleshire, and the Inner Hebrides, 

 the fineness of grain of the outer band culminates in a perfect volcanic glass. Where 

 this occurs, the glass is usually jet black, more rarely greenish or bluish black in tint, 

 and varies in thickness from about half an inch to a mere varnish-like film on the outer 

 face of the dyke, the average width being probably less than a quarter of an inch. On 

 their weathered surface, these external glassy layers, generally present a pattern of rounded 

 or polygonal prominences, varying up to four or five lines or even more in diameter, and 

 separated by depressions or narrow ribs that remind us of the lines seen in perlitic 

 structure. The transition from the glass to the crystalline part of the marginal fine- 

 grained strip is usually somewhat abrupt, insomuch that on weathered faces it is often 

 difficult to get good specimens, owing to the tendency of the vitreous portion to fly off 

 when struck with the hammer. The glass doubtless represents the original condition of 

 the rock of the dyke. It was suddenly chilled and solidified by contact with the walls 

 of the fissure. Inside this external glassy coating, the molten material had time to 

 assume a more or less completely crystalline condition before solidification. Not 

 infrequently the glass shows spherulitic forms, visible to the naked eye, and likewise 

 a more or less distinctly developed perlitic structure. These features, however, are best 

 studied in thin sections of the rock with the aid of the microscope, as will be 

 subsequently referred to. 



In some dykes, the glass is not confined to the edges, but runs in strings or 

 broader bands along the central portions. I have found several examples of this 

 peculiarity. The most remarkable of them occurs in the well-known dyke of Eskdale, 

 which runs for so many miles across the southern uplands of Scotland.* This dyke 

 throughout most of its course is a crystalline rock of the less basic type. At Wat 

 Carrick, in Eskdale, it presents an arrangement into three parallel bands. On either side 

 lies a zone about eight feet broad of the usual crystalline material. Between these two 

 marginal portions there is an intercalated mass sixteen to eighteen feet broad, of a very 

 compact and more or less vitreous rock. The demarcation between this central band and 

 the more crystalline zones of the outside is quite sharp, and the two kinds of rock show 

 a totally distinct system of jointing. There can, therefore, be little doubt that the 

 glassy centre belongs to a later uprise than the outer portions, though possibly it may 

 still have been included in the long process of solidification of one originally injected 

 mass of molten material. 



Mr C. T. Clough, while mapping for the Geological Survey the extraordinarily 

 numerous dykes in the eastern part of Argyleshire between the Firth of Clyde and 

 Upper Loch Fyne, has observed six or seven examples of dykes showing glassy 

 bands in their centres, with characters similar to those of the Eskdale dyke. He 

 informs me, that he has found an absence of definite and regular joints in the 

 central glassy band, and on the other hand, an irregular set of divisional planes by which 

 the rock is traversed, and which he compares to those seen in true perlitic structure. 



* See Proc. Roy. Phys. Soc. Edin., v. (1880) p. 241. 



