360 G. W. Tyrrell — Intrusions of Kilsyth-Croy District. 



able partlj^ or wholly to enclose crystals of augite. There is no 

 evidence of two separate generations of felspar. The crystallization 

 of the felspar was a perfectly continuous process, the composition 

 varying from labradorite at the beginning to acid oligoclase at 

 the end. 



The earlier felspars, augite, and iron-ores are perfectly idiomorphic 

 towards the micropegmatitic ground-mass, but the later felspars, as 

 explained above, appear to be continuous with the felspar of the 

 micropegmatite. This is specially noticeable in the coarser varieties. 

 The micropegmatite is in all probability the eutectic mother-liquor 

 left after the separation of the more basic elements.^ In an instructive 

 study of similar rocks from India, Sir Thomas Holland has suggested 

 that in a large boss, where the separation of the basic element is 

 comparatively small, the mother-liquor may crystallize as a large 

 body of granophyre. He would thus explain the frequent association 

 of gabbro and granophyre.^ It would also go far to explain the 

 frequently puzzling and contradictory relations of these two rocks, 

 which in the case of Skye led several careful observers to take 

 conflicting views as to which of the rocks was intrusive in the other. 



The Acid Veins. 

 A thin section of an inch vein from Colzium Quarry, Kilsyth, 

 shows under the microscope an apparently homogeneous mass of 

 kaolinized felspars, with minute grains of quartz, diversified by a few 

 sporadic crystals of augite, many ragged flecks of a deep-green chlorite, 

 and small skeleton-crystals of ilmenite. Between crossed nicols the 

 rock is seen to consist essentially of an interlocking aggregate of 

 small, stumpy felspars, highly altered, but still showing lamellar 

 twinning. The decomposed condition of the crystals prevents a 

 determination of the extinction angle, but by the application of the 

 Becke method to small areas of comparatively unaltered felspar in 

 contact with balsam, it can be shown that the refractive index is 

 lower than that of balsam. The felspar therefore in all probability 

 belongs to albite. This is in accord with the results of Elsden and 

 Falconer on the similar rocks of St. Davids and Bathgate Hills 

 respectively. These veins are similar to many aplites and micro- 

 gfanites. Petrographically they are soda-aplites or soda-microgranites. 

 In the larger veins quartz and micropegmatite are rare, but with 

 a decrease in the width of the veins there is a corresponding increase 

 in the proportion of these constituents, and the rock becomes more and 

 more like the intersertal material in the diabase. 



The Contact Rocks. 

 Towards the margin the structure becomes more and more ophitic, 

 the felspars become more lathy, the proportion of iron-ore increases, 

 micropegmatite and quartz dwindle and disappear, and the rock finally 

 takes on a thoroughly basaltic structure. The felspar, however, 

 remains of the same species — medium labradorite — as in the coarser 

 rocks, and the increasing basicity towards the margin must be 



' Teall, Brit. Petrog., 1888, p. 401. 

 - Holland, Q.J.G.S", 1897, vol. liii, pp. 412-14. 



