448 PK0E.J. W. JUDD AND G. A. J. COLE ON THE 



gradually diminishes as we trace it towards the interior of the dyke, 

 the glassy rock merging insensibly into the basalt. We shall point 

 out later the peculiar columnar and other joint-structures which are 

 exhibited at this locality. 



At Lamlash (Holy Isle), Arran, the mode of occurrence has been 

 very clearly described and illustrated by Macculloch, who gives a sec- 

 tion at the spot*. The glassy selvage of the dyke never exceeds one 

 inch in width ; it is not so perfectly vitreous as that of the Beal 

 near Portree, its lustre inclining more to resinous, and it merges 

 insensibly into the basalt of the body of the dyke. 



At Some Point, on the north-west coast-line of the Isle of Mull, a 

 very vitreous selvage is found in a dyke that intersects the lava- 

 streams composing the great plateau of Mishnish ■ it seldom exceeds 

 one fourth of an inch in width, and passes into the basalt of the 

 dyke by the most insensible gradations. 



At Gribun, in the west of Mull, a dyke of basalt is seen by the 

 road that leads from the village of that name to Kilnnichen, just 

 where it emerges on the basalt-plateau; and the sides of this dyke 

 are formed by glassy selvages never more than half an inch in 

 thickness. 



Lastly, at Screpidale, in the Isle of Eaasay, one of the dykes of 

 basalt passing through the grand precipices of Jurassic strata has 

 also glassy sides. The actually vitreous portions of this dyke, how- 

 ever, are very thin indeed, forming little more than surface-films. 



The only other case which we have found in the Western Isles of 

 Scotland at all comparable to these, is the occurrence of a basic glassy 

 rock among the later-formed products of the volcano of Beinn Shiant 

 in Ardnamurchan. Unfortunately the specimens were not found 

 in situ; and their exact mode of occurrence is therefore unknown. 

 Prof. Jameson t and Dr. A. Geikie t have described the occurrence 

 of a " pitchstone " at Eskdale which appears to be similar in com- 

 position to the rock of Beinn Shiant. Both are probably augite- 

 andesite glasses. From the description given of the pitchstone of 

 Eskdale it would appear as though a columnar dyke had opened 

 along its central plane of weakness, where the two sets of columns 

 meet, and permitted the extrusion of material which consolidated 

 as a " pitchstone." 



• In the north of Ireland some of the basaltic dykes have been 

 described as having vitreous selvages§. The microscopic characters 

 of one of the basaltic glasses, that of Slievenalargy, co. Down, have 

 been described by Mr. Eutley ; and it has been analyzed by Dr. 

 Haughton||. 



3. Specific Gravity. 

 As Delesse so long ago pointed out, the specific gravity of a rock 



* Western Islands of Scotland, vol. ii. p. 437. 



t Mineralogical Description of Dumfriesshire, p. 115. 



\ Proceedings of the Eoyal Physical Society of Edinburgh, vol. v. p. 29. 



§ Kinahan, Geol. Mag. decade ii. p. 426. 



|j Journal of the Eoyal Geological Society of Ireland, vol. iv. p. 227. 



