Vol. 52.] BASALT- PLATEAUX OF NORTH-WESTERN EUROPE. 387 



y-axis of optic elasticity makes a small angle with the twin- 

 line. These characters agree with those of epistilbite. In other 

 parts of the same large amygdnle the epidotc-crystals are imbedded 

 in what seems to be a felspar. This latter mineral is rather 

 obscure, and twin-lamellation is rarely to be detected ; but it seems 

 highly probable that felspar has here been developed by meta- 

 morphic agency at the expense of zeolites which once occupied the 

 araygdule. I have observed undoubted examples of this in meta- 

 morphosed basalts from other parts of Skye, e. g. from Creagan 

 Dubha, near the granophyre-mass of Beinn Dearg. 1 The felspar 

 occurs there in the same fashion, and in the same relation to 

 epidote [2700, 2701]. In the specimens now described the chief 

 minerals in the metamorphosed amygdules are those already named : 

 others occur more sparingly, associated with them. In some cases 

 there is a grass-green, strongly pleochroic, actinolitic hornblende, 

 accompanied by a little iron pyrites [6615]. 



1 Epidote and various hornblendic and augitic minerals are 

 characteristic products in the metamorphism of amygdaloidal basalts 

 in other regions : felspar with this mode of occurrence I have not 

 seen except in Skye, where it seems to connect itself naturally with 

 the abundance of zeolites in the amygdules of the non-metamorphosed 

 lavas. It is to be observed that in these basalts from Loch Scavaig 

 the alteration is shown especially in the amygdules, the body of the 

 rock not being greatly affected : this indicates a not very advanced 

 stage of metamorphism. The production of uralitic hornblende, 

 rather than brown mica, from the augite and its decomposition- 

 products seems to be characteristic of the metamorphism of basaltic 

 as distinguished from andesitic rocks, and is well illustrated by com- 

 parison of the two sets of lavas near the Shap Granite.' 2 



A re-examination of parts of the gabbro mountains of Rum has 

 shown me that, though in a less marked degree than in Skye, the 

 same banded structure may be detected in the thick beds or sills of 

 which these eminences are composed. The remarkably schist-like 

 bed of troctolite formerly described by me 3 lies between more 

 massive sheets that show a much ruder parallel structure. The 

 whole mountain of Allival overlying this troctolite is built up of 

 successive parallel sheets of gabbro, among which banding is of 

 frequent occurrence, the layers varying from less than an inch to a 

 foot or more in breadth, and lying parallel to each other and to the 

 upper and under surfaces of the sheets in which they occur. An 

 occasional example of curvature in the banding may be observed. 



Compared with the gabbros of the Cuillin Hills, those of Rum 

 display a similar but less definite aggregation of their component 

 minerals in definite layers or bands. In particular, the pyroxene 

 and olivine, either separately or together, are crowded along parti- 

 cular bands of darker hue, while the paler bands between them are 

 composed chiefly of felspar. The crystals or crystalline kernels are 



1 Compare Trans. Roy. Soc. Edinb. vol. xxxv. (1888) p. 166. 



2 Quart. Journ. Geol. Soc. vol. xlix. (1893) p. 361. 



■ Trans. Row Soc. Edinb. vol. xxxv. (1888) p. 123 ; Quart. Journ. Geol. Soc. 

 vol. 1. (1894) p. 649. 



Q. J. G. S. No. 206. 2 d 



