DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



179 



new facts and inferences, obtained from a recent re-examination of the locality. The 

 o-eneral form of the ridge of the Scuir will be understood from the accompanying map of 

 the island of Eigg (fig. 62). The length of the ridge is two miles and a quarter, its 

 Greatest breadth 1520 feet, its extreme height 1289 feet. It consists of successive sheets 

 of columnar porphyritic pitchstone, and of a dull grey de vitrified felsitic rock, the whole 



Fig. 62.— Geological Map of the Island of Eigg. P, Pitchstone of Scuir ; R, old river gravel under pitchstone ; pp, small veins 

 of Pitchstone; bb, dykes, veins and sheets of intrusive basalt; the short black lines running N. W. and S.E. are basalt dykes ; 

 ff, granophyre sills; D, bedded basalts with occasional tuffs; F, sheet of the " pale group " of Mull; 1, 2, 3, 4, clays, shales, 

 sandstones, limestones, &c. (Jurassic) ; xx, Loch Beinn Tighe ; x, Loch a Bhealaich. :»— >- General dip of the rocks. 



having a united thickness of several hundred feet. Examined in thin sections under the 

 microscope, the vitreous beds present a pale brown glass, with abundant depolarising 

 microlites, large porphyritic crystals of sanidine, sometimes plagioclase, and smaller 

 crystals of augite and magnetite. Some of these beds have only a feebly resinous lustre, 

 and resemble in some respects andesites. In one of the slides the porphyritic felspars 



