DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 129 



But by the time these rocks have reached this valley, they have already lost, their 

 usual brown colour and crumbling surfaces, and have assumed the indurated splintery 

 character, though still showing their amygdaloidal structure. They are much traversed 

 by veins and strings of felsite and quartz-porphyry, which rocks at last appear as a broad 

 band that runs up the bottom of the Coire Uaigneich, and ascending the col, crosses it 

 south-westwards into the Glen nan Leac. On the left or south-eastern side of this intrusive 

 mass, a portion of Lower Silurian quartzite and limestone (here and there altered into 

 white marble) is traceable for several hundred yards up the stream.* Whether this is 

 really in place, and projects as the top of an eminence round and over which the volcanic 

 rocks were accumulated, or whether it is a mass that has been torn away and carried 

 upward during some of the paroxysms of eruption, I could not determine. Knowing 

 how large are the portions of schist embedded between the basalts of Mull, I do not 

 think the great size of this mass necessarily precludes us from regarding it as displaced 

 in the same way. "Where the quartzite and limestone first appear at the lower part of 

 the valley, they present an interesting example of that sheared structure with which 

 recent investigations in the North- West Highlands have now made us familiar.t The 

 two rocks have been ground into each other so as to produce a compound that is neither 

 limestone nor quartzite, but a calcareous quartzose schist, in which the beautifully parallel 

 planes of division that mark the surfaces of movement bear the closest resemblance to flow- 

 structure. There can be no doubt that here in the midst of the Tertiary volcanic masses, 

 and not improbably as a result of volcanic explosion, there is revealed to us the existence 

 underneath this district of one of those great thrust-planes in the Silurian rocks which 

 have had so powerful an influence in the production of the younger schists of the 

 Highlands. The evidences of metamorphism and the formation of schists by mechanical 

 movement after the Lower Silurian period, so abundant in the north of Sutherland, 

 are thus found to continue southward even across the island of Skye. 



The bedded basalts of Strathaird, after dipping down towards the N.N.W., bend up 

 where they are interbanded with dolerites and gabbros, and form the prominence called 

 An Stac, which rises as the eastern boundary of the Coire Uaigneich. Their steep dip 

 away from the mountain is well seen from the east side, and their outward inclination is 

 continued into the ridge to the southward. Similar rocks appear on the other flank of 

 the band of quartz-porphyry, and form the base of Blath Bheinn. The bedded basalts are 

 everywhere of the usual altered, indurated, and splintery character. The intrusive sheets 

 interposed between them become thicker and more abundant higher up, until they con- 

 stitute the main mass of the hill. But that they are in separate sheets, and not in one 

 amorphous mass, can be recognised by the parallel lines that mark their boundaries. 



The quartz-porphyry sends out veins into the surrounding rocks, and is obviously the 

 youngest protrusion of the locality, except of course the N.N.W. basalt-dykes which 



* This limestone was noticed by Von Oeynhausen and Von Dechen, but they believed it to be a portion of the 

 Lias torn off and carried upward by the eruptive rocks (Karsten's Archiv, i. p. 79). 



t This rock was first recognised by Mr H. M. Cadell, who accompanied me in one of my excursions over the ground. 



