Yol. 52.] BASALT-PLATEAUX OF NORTH-WEST ERN EUROPE. 385 



descends abruptly across these basalts and also across the Torridon 

 Sandstone, on which they unconformably rest. These two groups 

 of rocks are not only truncated by the gabbro, but are traversed by 

 the intricate system of sills, dykes, and veins already referred to. 



Where it abuts against the sandstones and basalts, the gabbro is 

 arranged in vertical bands of different mineral composition and 

 texture. Much of it is remarkably coarse, some bands displaying 

 pyroxene- crystals more than an inch in length. There is no fine- 

 grained selvage here indicative of more rapid cooling. So coarse, 

 indeed, is the rock close up against the sandstone that the junction- 

 line can hardly be supposed to be the normal contact of the intrusive 

 rock. This inference is confirmed by the existence of a singular 

 kind of breccia between the gabbro and the sandstones. It is a 

 tumultuous mass of fragments of coarse and fine gabbro, Torridon 

 Sandstone and Shale, and plateau-basalts, imbedded in a pale crys- 

 talline matrix of fine granular granophyre. Veins from this acid 

 intrusion run off into the gabbro on the one side as well as into 

 the Torridon Sandstone on the other. It would seem that this 

 junction-line has been one of great movement, that the gabbro- 

 sheets have subsided against a fault-wall of plateau-basalt and 

 Torridon Sandstone, and that subsequently an intrusion of finely 

 granular granophyre has come up the fissure, involving in its ascent 

 fragments of all the materials around. 



The rocks for a considerable distance to the south of the gabbro 

 are intensely altered. The Torridon Sandstone has been so indu- 

 rated as to pass into a bleached white quartzite, while the shales in- 

 terstratified with it have been converted into a kind of porcellanite. 



But the most interesting alterations are those to be observed in 

 the plateau-basalts which, at a height of about 300 feet above the 

 sea, are to be seen in nearly horizontal sheets that lie immediately 

 on the upturned edges of the Torridon Sandstone. These lavas 

 have suffered great metamorphism : their alternations of amygda- 

 loidal and more compact sheets can still be recognized, though their 

 enclosed amygdules have in places been almost effaced. They show 

 the dull, indurated, splintery character, with the white weathered 

 crust, which I formerly described as distinctive of this type of 

 contact-metamorphism, and are traversed by numerous sills and 

 veins of gabbro. No large mass of granophyre appears here at the 

 surface. We can hardly be mistaken in looking upon this altera- 

 tion as due either to the influence of the main body of the gabbro, 

 or perhaps more probably to the abundant acid sills, dykes, and 

 veins, possibly to botb causes combined. It must be admitted that 

 there may be a considerable body of granophyre underneath the 

 locality, the surface-dykes and veins being indications of its vicinity. 



In my former memoir I dwelt upon the remarkable alteration 

 of the plateau-basalts as they approach the large masses of gabbro 

 or granophyre. 1 During the summer of 1895 Mr. Harker, in the 



1 Trans. Eoy. Soc. Edin. vol. xxxv. (1888) p. 167. Prof. Judd has referred 

 the alteration of the rocks to solfataric action (Quart. Journ. Geol. Soc. vol. xlvi. 

 1890, p. 341). I have been unable to detect any evidence of such action. The 

 alteration is always intimately connected with the presence of intrusive iu;i?s<s, 



