1871.] 



GEIKIE TEEXIAEY VOLCA>fIC ROCKS. 



299 



Side of Beinn Tighe, Eigg. 



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 tex- 

 ture 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 tra- 

 verses the basalts on the east side of the Beinn Tighe — one of the 

 outlying hUls 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 tachy- 

 lite into the surrounding interbedded basalt (fig. 5). The marginal 

 crust of tachylite varies in thick- 

 ness in different veins, ranging from Fig. 5. Plan of Basalt Veins 

 one-third to about one-eighth of with Tachylite edges, East 

 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 encoun- 

 tered ; in several slices of it which 

 I have had prepared for microscopic 

 examination and reduced to ex- 

 treme thinness, I am unable to get 

 any light sent through, even at the 

 edges. 



The veins run vertically, horizon- 

 tally, or at any angle, and branch 

 or unite, sweU 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 dis- 

 tinguished 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 



