BASALT-GLASS OF THE WESTERN ISLES OE SCOTLAND. 451 



gonites and other . hydrated materials of that class that it serves 

 as an easy means of distinction between the altered and unaltered 

 forms. 



The whole of the basalt-glasses are strikingly magnetic. The 

 powder of all of them is easily attracted by the magnet, those 

 of Some, Screpidale, and Gribnn exhibiting this character in a 

 specially marked degree. 



Basalt-glass is at once distinguished from vitreous substances of 

 more acid composition by its easy fusibility, the product being an 

 opaque black-brown or black bead. The fusibility of the varieties 

 from the Western Isles appears to be little above 2 of von Kobell's 

 scale, while that of typical obsidian is no less than 4*5 of the same 

 scale, the product of fusion being a white enamel. 



The basalt-glass from the Hebridean localities appears to be 

 always traversed by numerous joint-planes. The most pronounced 

 of these are parallel to the side of the dyke, and enable the glassy 

 material to be easily separated from the basalt in thin la} : "ers. 

 Besides these principal joints, there are usually one or two other 

 sets at right angles to one another and to the sides of the dyke. 

 Hence the surfaces of the dyke, where in contact with the rock 

 which it traverses, are broken up by these joints into small rect- 

 angular areas. This character is strikingly exemplified by the 

 basalt-glass of the Beal. 



The same dyke also exhibits in some portions of its glassy selvage 

 a remarkable columnar structure. The columns are very minute, 

 often as thin as fine needles, about 1| inch in length, and some- 

 times beautifully curved. Some specimens present a miniature 

 reproduction of the celebrated Clam-shell Cave in Staffa. Occa- 

 sionally the columns are as much as ^ inch in diameter (PI. XIII. 

 fig.l). 



This finely columnar structure affords an admirable illustration 

 of the fact that the diameter of the columns in a rock is in part 

 dependent on the coarseness of the materials of which it is com- 

 posed. "While the highly crystalline rocks of the Shiant Isles and 

 Ailsa Craig form columns with a diameter of 10 or 12 feet, these 

 vitreous rocks exhibit columns of almost microscopic dimensions. 



Besides the rectangular and columnar jointing, a minutely 

 spheroidal structure is sometimes found developed in the basalt- 

 glass. Hence, under the microscope, it frequently exhibits traces 

 of the well-known "perlitic" structure. This last-mentioned fact 

 has been noticed by Zirkel in the case of the basalt-glass of Maros- 

 tica (Monte Glosso) in the Euganean Halls, by Mr. Rutley in the 

 "tachylyte" of Slievenalargy in the north of Ireland, and by 

 Fouque and Levy in the lava of Graham's Isle. 



5. Solubility in Acids. 



So much importance has been attached by some authors to this 

 character, as presented by rocks of this class, that it is perhaps 

 desirable to discuss in some detail the behaviour of the rocks now 

 being described when subjected to the action of acid. 



