Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/439

 1. Basalt, Anamesite and Dolerite Veins. — These are closely connected both with the dykes and with the intrusive sheets, into either of which any vein may pass, or from which any vein may proceed. They commonly consist of a very compact finely crystalline rock, often paler in colour than that of the interbedded basalt-rocks, even where these are most close-grained. These features may be well seen along the coast-sections to the north of Kildonan. Among the veins of that as well as of other localities, a minutely amygdaloidal texture is occasionally observable, the small kernels being arranged in lines parallel with the sides of the vein and most marked along its centre. The grain of the rock usually becomes very close towards the edge of the vein, passing sometimes through various stages of flinty basalt into bright black lustrous tachylite. The most perfect example which I observed of this difference between the texture of the central and outer parts of a vein occurred in a vein which traverses the basalts on the east side of the Beinn Tighe — one of the outlying hills of the Scur ridge. The rock is of a dark, very fine- grained basalt, which along the walls of the vein assumes a vitreous aspect, and sends out a loop or thread of black pitchstone-like tachylite into the surrounding interbedded basalt (fig. 5). The marginal crust of tachylite varies in thickness in different veins, ranging from one-third to about one-eighth of an inch. Sometimes it shades into the basalt within ; in other cases it forms a pellicle, which cracks off in weathering. It is one of the most opaque rocks I have ever encountered ; in several slices of it which I have had prepared for microscopic examination and reduced to extreme thinness, I am unable to get any light sent through, even at the edges.

The veins run vertically, horizontally, or at any angle, and branch or unite, swell out or diminish, in a capricious manner. Their close texture and abundant joints make them weather differently from the rocks which they traverse. This, added to a frequent difference of colour, renders them a conspicuous figure along the coast-cliffs of Eigg (see fig. 6). Some striking illustrations occur on the east side of the island north of Kildonan, and also on the great precipice below Bideann Boidheach, where the pale thread-like veins may be distinguished even from a distance as they rise along the sombre face of the cliffs.

2. Pitchstone and Felstone Veins. — Although nearly the whole of the veins in Eigg are protrusions of doleritic rock, there occur a few in which the rock is pitchstone and, in one case at least, felstone. That these veins are, on the whole, later than those just described

Fig. 5. Plan of Basalt Veins with Tachylite edges, East Side of Beinn Tighe, Eigg.