185 



in the Castle Grounds at Brodick, in the Isle of Arran, and at the Quiraing 

 in Skye. (1) The basalt-glasses are distinguished -from the andesite- and 

 liparite-glasses, which they closely resemble in external characters, by their 

 high specific gravity and easy fusibility. The following table gives the 

 specific gravities and silica percentages of the Scotch representatives of 

 this group according to Messrs. JUDD and COLE : 



Sp. Gr. SiO 2 . 



The Beal, near Portree ... 272 ... 52-59 /> 



Lamlash, Arran ... ... 278 ... 56'05 



Gribun, Mull... ... ... 2'82 ... 50*51 



Bcrepidale, Eaasay ... ... 2'84 ... 46*68 



Some ... ... ... 2-89 ... 47'46 



Brodick ... ... ... 2-83 ... 53-96 



The specific gravity of the natural glass, in the~majority of cases, bears 

 the same relation to the associated basalt as that of the artificial glass to 

 the rock from which it is obtained by melting. A remarkable and unex- 

 plained exception to this rule, first noticed by DELESSE, occurs in the case 

 of the Lamlash dyke. The specific gravity of the basalt is 2'649 accord- 

 ing to DELESSE and 2 - 67 according to Mr. DAVIES ; that of the glass is 

 2714 according to the former observer, and increases in passing outwards 

 from 272 to 278 according to the latter observer. Prof. JUDD points out 

 that the rock of the dyke is a magma-basalt in which only incipient 

 crystallization has taken place, and suggests that the anomaly may be 

 due to the fact that the central portions have been more altered than 

 the exterior. The lustre of the basalt-glasses varies from resinous to 

 vitreous. Sometimes, as in the case of the Beal mass, the glass shows a 

 minute columnar jointing. 



In thin sections the basalt-glasses above referred to are characterized 

 by great opacity, due to the presence of finely distributed magnetic dust. 

 Sometimes this dust is concentrated in nebulous masses of a more or less 

 spherical form and then the isotropic and truly glassy character of the 

 intervening substance can be ascertained. Perlitic structures may 

 occasionally be observed. Porphyritic felspars frequently occur in the 

 glassy matrix and these are usually fragmentary and completely honey- 

 combed with inclusions. Olivines are occasionally present and, in the 

 case of the Lamlash occurrence, augite has also been observed. Crystallites, 

 microlites, and skeleton crystals are common and increase in amount and 

 definiteness until the rock becomes a magma-basalt. 



(1) Noted by DR. HEDDLE. M.M., Vol. V., p. 8. 



