384 SIR A. GEIKIE ON THE TERTIARY [May 1 896, 



VI. The Intrusive Gabbros. 



Some of my more recent observations among the gabbros of Skye 

 have already been communicated to the Society. 1 In conjunction 

 with my colleague Mr. Teall I have described a remarkable 

 banded structure traceable in these rocks, wherein the component 

 minerals have crystallized along different layers in such a manner 

 as to present a singular resemblance to the arrangement charac- 

 teristic of many Archaean gneisses. Further investigation last 

 summer has shown me that this banding is extensively developed 

 in the Cuillin Hills. The mountains that surround the head 

 of Loch Scavaig and sweep round Loch Coruisk up to the great 

 crests of Sgiirr na Banachdich everywhere display on their bare 

 black crags a distinct bedded structure. 



On the eastern side of Loch Scavaig the rock presents a rudely- 

 banded character, the bands or beds being piled over each other 

 from the sea-level up to the summits of the rugged precipices, and 

 dipping into the hill at angles of 25° to 35°. Abundant dykes 

 and veins of various basic, intermediate, and acid rocks cut this 

 structure. The individual layers here, as at Druim an Eidhne, 2 are 

 sometimes wavy and puckered. 



Even from a distance the alternating lighter and darker beds can 

 readily be seen, so that the banded structure, with the variations in 

 its inclination, may be followed from hill to hill. The regularity of 

 the arrangement, however, is often less pronounced on closer 

 inspection. While the gabbro is rudely disposed in thick beds, 

 indicative of different intrusive sheets or sills, with which the 

 banding is generally parallel, considerable irregularities may be 

 observed in the arrangement of the structure of individual sheets. 

 These sheets may be parallel to each other, and yet, while in some 

 the banding is tolerably regular in the direction of the planes of 

 the sheets, in others it is much twisted or inclined at various angles. 



On the western side of the Coruisk river the banding is vertical ; 

 southward from that stream it inclines slightly towards the south, 

 but soon again becomes vertical, and continues conspicuously so at 

 the junction of the gabbro with the Torridon Sandstones and the 

 plateau-basalts on the western side of Loch Scavaig. 



In the great corries and ridges of the Cuillin Hills traces of 

 bedding are generally to be recognized, with later sills injected at 

 different horizons and in different directions. Instead of being one 

 great eruptive boss, the gabbro of this district is in reality an 

 exceedingly complicated network of sills, veins, and dykes. While 

 the general inclination of the bedding sometimes continues uniform 

 in direction and amount from one ridge to another, it is apt to 

 change rapidly, as if the complex assemblage of intruded masses 

 had been disrupted and had subsided in different directions. 



The gabbro overlies the bedded basalts of the plateau all the way 

 from Glen Brittle to the western side of Loch Scavaig. It then 



1 Quart. Journ. Geo). Soc. vol. 1. (1894) pp. 216, 645. 



2 Op. cit. p. 648, and pi. xxvi. 



