362 PROF. J. W. JIJDD ON THE TEETIAET AND 



§ 3. MiKEEALS OF WHICH THE EoCKS AEE BTJILT UP. 



As the species of minerals, and the different varieties of those 

 species, are the same in the peridotites as in the dolerites and gab- 

 bros of the Inner Hebrides, it will be necessary to give a detailed 

 account of them in the present memoir, the discussion serving not 

 only for the ultrabasic, but also for the basic rocks, which will be 

 described in a subsequent communication. 



Much confusion has been introduced into petrographical literature, 

 in consequence of all the characters presented by minerals being 

 treated as if they had precisely the same significance. But while 

 some of the characters of the rock-forming minerals are original 

 and essential, others are, as certainly, secondary and accidental. 

 Moreover, the minerals, since their first crystallization, may have 

 undergone several series of changes totally dissimilar in kind, and 

 resulting from causes altogether different. It therefore becomes 

 necessary to distinguish not only the secondary from the original 

 characters, but to assign each of the secondary characters to its 

 proper cause. This is the task which I have endeavoured to per- 

 form in the case of each of the minerals which go to build up the 

 rocks now under discussion. 



It will, I think, conduce to clearness, if I indicate at the outset 

 what appear to have been the original peculiarities of the several 

 minerals in these rocks, and show what were their physical proper- 

 ties and chemical composition in their unaltered condition. I shall 

 then proceed to describe the changes which, in certain situations, 

 they are found to have undergone, and to discuss the cause of these 

 changes. 



The Felspars. — Plagioclase felspar, which constitutes one half, or 

 even more than one half, of some of the gabbros, and is equally 

 abundant in the eucrites and in the troctolites, becomes so rare in 

 many of the peridotites that it can no longer be regarded as an 

 essential constituent of them. But, as already stated, we find every 

 possible gradation from the most highly felspathic rocks to others 

 in which the felspar entirely disappears. 



The crystals of the felspars vary greatly in size ; sometimes the 

 gabbros are fine-grained, almost compact rocks ; at other times they 

 are built up of crystals of great size, up to one or two inches in 

 length. As a rule, the most coarsely crystalline varieties are found 

 in the centre of the largest intrusive masses, like those of Skye and 

 Ardnamurchan. In the case of these large masses, the tint and 

 play of colours in some of the felspar-crystals suggest that they are 

 labradorite, and they were recognized as such by MaccuUoch as long 

 ago as 1819 *. 



The tests of specific gravity, of fusibility, of the flame-reaction of 

 Szabo, and of the extinction-angle, as suggested by Levy and Eouque, 

 all point to the conclusion that these felspars are, in chemical com- 

 position, intermediate between labradorite and anorthite, in some 

 cases approaching the former, in other cases the latter species. Such 



* A Description of the Western Isles of Scotland, vol. i. p. 413. 



