538 S. Allport — On the Igneous Rocks of Armn. 



exhibits douhle refraction. In ordinary liglit, there are seldom any 

 distinct indications of individiial forms, the appearance being rather 

 that of a confused imperfectly blended mass of glassy materials. 

 Its true character is, however, well seen in polarized light; the axes 

 of the Nicols being parallel, there are still comparatively few sharp 

 outlines, but as either of the prisms is rotated, the mass appears to 

 break up into variously coloured little patches, which gradually 

 assume a more definite form as the axes approach to a right angle ; 

 in that position it has the appearance of a granular compound of 

 crystalline fragments, among which there may be seen, in most 

 cases, a few more or less perfectly formed crystals. It is not, 

 however, a granular compound in the proper meaning of the term : 

 it is evidently a mass which has been consolidated whilst in an in- 

 cipient stage of crystallization ; or, in other words, an originally 

 homogeneous mass has undergone a certain amount of molecular 

 change, insufficient for the development of crystalline forms, but 

 sufficient for the production of double refraction. 



It would appear from these facts, that in the case of pitchstone, 

 the conditions under which the consolidation of the matrix took 

 place were altogether unfavourable to crystallization ; but that in 

 the case of a felsitic base, the process of crystallization had com- 

 menced, and had been arrested, possibly by the act of consolidation. 

 But it is by no means obvious in what way the conditions differed, 

 for it is quite certain, that in both cases, felspar and quartz crys- 

 tallized out from the mass, and in doing so caught up portions of 

 the surrounding glassy or felsitic matrix. The explanation may 

 however lie in the fact, that there is more silica in the felsitic than 

 in the vitreous parts of these rocks. Concurrently with an incipient 

 crystallization there has also been a partial formation of dis- 

 tinct constituents ; for in most cases, if not in all, quartz may be 

 distinctly recognized. Although the peculiar texture just described 

 may be regarded as the chief characteristic of a large class of rocks, 

 it is only in typical specimens that it exclusively prevails, for it 

 will be seen, that by a gradual increase in the quantity of granular 

 and crystalline particles, it passes into a more or less perfectly 

 crystallized structure. It will shortly be shown that felsitic sub- 

 stances frequently occur in pitchstone, and in the following de- 

 scriptions the term will always be used to indicate the structure 

 just explained. 



On the west coast, between Drumadoon Point and the headland 

 called Cleitadh nan Sgarbh, the cliffs round the bay consist of red 

 sandstones traversed in places by veins of pitchstone and red jaspery 

 "hornstone." One of them near King's Cove is a brown pitchstone, 

 forming a vein high up in the cliff, and is a rock of special interest, 

 as it is very variable in structure, and shows clearly that ordinary 

 pitchstone, and the compact red "hornstone," are simply modi- 

 fications of one and the same mass. Four specimens taken from 

 this vein exhibit the following varieties : — 



1. A yellowish-brown pitchstone, full of small round grains and 

 narrow bands, differing slightly in colour from the glassy base. 



