OF TEE AVESTERX ISLES OE SCOTLAND. 347 



everywhere in the Western Isles the rock-surfaces are acted upon 

 by peaty waters ; and just as brown sandstones and other fer- 

 ruginous rocks become bleached by this action, so do the igneous 

 masses, which have been rendered more permeable by the extensive 

 alteration of their constituent minerals, become bleached to the 

 depth of a few millimetres from the surface. That this is the true 

 explanation of the phenomenon is confirmed by a microscopic study 

 of these white crusts themselves, and of the parts of the rock where 

 they graduate into the dark-coloured mass. 



Another distinguishing characteristic of the Scottish propylites is 

 the presence of considerable quantities of pyrite and other sulphides 

 diffused through their mass. Certain rocks about Salen in Mull, 

 and in Ardnamurchan, for example, when broken, are seen to be 

 studded all through their mass with pyrite-crystals. In other cases 

 marcasite and chaleopyrite are found, and these sulphides are often 

 present in such quantity as to constitute an important constituent 

 of the rocks. All who are acquainted with the " greenstone-tra- 

 chytes " of Eastern Europe and the " propylites " of the Western 

 Territories of the United States will recognize this as a feature 

 which they present in common with the Scottish rocks. In one case 

 I have found metallic copper forming thin plates scattered through 

 the rock. 



One of the readiest means of distinguishing the dark-coloured, 

 much-altered andesites from the overlying olivine-basalts is by a 

 determination of their specific gravity. While the gabbros, dolerites, 

 and basalts have a density which always approaches to and some- 

 times exceeds 3, the propylites and their deep-seated representatives 

 have a distinctly lower specific gravity, ranging from 2-4 to 2-9 *. 



During the last fifteen years, I have been able to compare the 

 results of a great number of determinations of the specific gravities 

 of the Scottish propylites, which have been carried out by various 

 methods, in the geological laboratories of the JN'ormal School of 

 Science and Royal School of Mines. Some of these determinations 

 have been kindly made for me by Mr. Grenville Cole, F.G.S., or by 

 students working under his supervision ; others have been contri- 

 buted by Mr. W. B. D. Edwards, Mr. T. H. Holland, Ui\ J. W. 

 Evans, r.G.S.,and Mr. W. E. Hume, E.G.S., working in the Research 

 Laboratory of the school, and these gentlemen have spared no pains 

 in obtaining and verifying their results. 



A comparison and analysis of the great mass of specific-gravity 

 determinations thus placed at my disposal shows that the propj-lites 

 of the Western Isles of Scotland may be roughly classed in two 

 great groups — those in which the specific gravity ranges from 2*4 to 



* Shortly after the appearance of my first memoir on those rocks in 1874, 

 Mr. W. Walker, F.G-.S., wrote to inform me that, having visited tlic district 

 described by me, in company with Mr. James Durlmm, ho had been led to devise 

 a portable balance for the purpose of obtaining rapid and approximately accu- 

 rate determination of I be specific gravity of rock-speciraons. This balance proves 

 to bo of great service, as it can be employed by the geologist when he is far 

 away from the resources of a laboratory. (See Geol.Mag. dec. ii. vol. x. p. lOi), 

 and'Proc. Geol. Assoc, vol. viii. p. 278.) 



