DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 117 



and there are instances where this cannot be shown not to be the case, I have obtained 

 so many proofs of the invasion of the basic by the acid rock that I have no doubt the 

 former is as a general rule the older of the two. The Suisnish headland exhibits the 

 structure represented in fig. 14. For about 300 feet above the sea-level the steep grassy 

 slope shows outcrops of the dark, sandy shales and yellowish brown, shaly sandstones 

 of the Lias which form the range of cliffs to the eastward. These gently inclined 

 strata are cut through by many vertical basalt-dykes, some of which intersect each other, 

 but among which by far the largest is the mass shown in the figure. This broad dyke 

 consists of a dolerite, the largely crystalline texture of which marks it off at once from 

 the others, which are of the usual dark, heavy, fine-grained type, with an occasional 

 andesitic and porphyritic variety. Traced up from the sea-margin, the dyke loses itself 

 in a talus of blocks from the cliff above, so that its actual junction with the mural front 

 of the sill cannot be seen. But that it joins that mass, with which it agrees in petro- 

 graphical characters, hardly admits of question. The cliff consists of a thick sheet of 

 coarsely crystalline dolerite (c in fig. 14), which in its general aspect at once recalls the rock 

 of Fair Head. It varies considerably in texture, some parts of the mass are exceedingly 

 coarse, like the Skye gabbros, and present a fibrous structure in their augite resembling 

 that of the diallage in these rocks ; other portions assume the compactness of basalt. A 

 specimen of medium grain under the microscope shows the typical ophitic structure so 

 generally found among the dolerites both of the plateaux and of the intrusive sheets. 

 This sill must be about 200 feet thick, and like the rock at Fair Head is traversed from 

 top to bottom by joints that divide it into prisms. It appears to bifurcate eastward, one 

 portion running with a tolerably uniform thickness of a few feet as a prominent band at 

 the top of the shales and sandstones, the other slanting upwards and gradually thinning 

 away in the granophyre. 



Towards its base, near the contact with the underlying shales, the rock as usual 

 becomes finer grained, and the thin band just referred to resembles in texture one of the 

 wider basalt-dykes. Westwards the rock can be followed round the top of the grassy 

 slopes formed by the decay of the shales. Though concealed by intervals of moorland 

 and peat, it is visible in the stream sections, and I think must be continuous, as a band 

 only a few yards thick, round the northern side of the hills as far as Beinn Bhuidhe, 

 where a similar sill makes a prominent crag. Its total area measures a mile and a quarter 

 in length by half a mile in breadth. The granophyre which overlies it forms part of an 

 interesting series of sheets which I have traced all the way from Suisnish to the braes 

 above Skulamus. 



Whether or not the whole sheet of basic rock is continuous, and whether it all 

 proceeded from the great Suisnish dyke, cannot be confidently decided, though from the 

 great thickness of the sill at the dyke, its attenuation outwards from that centre and its 

 uniformity of petrographical character, I am disposed to answer affirmatively. There is 

 no other probable vent to be seen in the neighbourhood, unless a massive dyke that runs 

 from Loch Fada north-westwards into Glen Boreraig can be so regarded. 



VOL. XXXV. PART 2. O 



