308 G. W. Tyrrell— Intrusions of Kilsyth- Croij District. 



show a distinct pleochroism from pale coppery red to a pale green. 

 The crystals are rendered prominent by a marginal alteration to a deep- 

 green, pleochroic, fibrous mineral, which probably bears the same 

 relation to hypersthene as bastite does to enstatite.^ The cracks 

 also are broadly outlined by the same substance. 



The abundant iron-ore offers some interesting features. It rarely 

 occurs in other than skeleton crystals. Frequently it is in large, flat 

 plates coating the faces of augite crystals. In one of the coarser 

 varieties from Durabuck Quarry, Croy, the thin cross-sections of plates 

 are dispersed all over the slide, and all conform to one of three 

 directions, at an angular distance of 120° from one another. The 

 appearance is of a large, discontinuous, triangular framework of 

 plates of iron-ore in which the other constituents have crystallized. 

 The form of this framework shows that this mineral belongs to the 

 ilmenite group. That it is titaniferous is put beyond doubt by the 

 occurrence in some of the more altered rocks of grey leucoxenic 

 decomposition products, and even yellow sphene. 



Beside the regular skeletal forai above described, there are other 

 curious forms. One is radial like the spokes of a wheel, each spoke 

 being knobbed at the extremity. There are also long saw-like 

 serrated rods. Other types take the form of irregular, closely packed 

 rods, the shape of the whole aggregate approaching to an idio- 

 morphic form. 



In addition to the constituents above described, a mesostasis always 

 occurs in the interspaces. In the coarser and fresher rocks this 

 material may be certainly identified as micropegrnatite. It forms 

 a regular intergrowth of rods of quartz and felspar, and where 

 contiguous to a felspar crystal the felspar of the micropegmatite is 

 continuous with it ; and the wave of extinction occasioned by the 

 chemical zoning of the felspar crystal starts in the micropegmatite. 

 In a coarse rock from Fankerton, near Denny, minute triangular 

 grains of quartz in regular rows are embedded in a ground-mass of 

 decomposed felspar. The micropegmatite also occurs as parallel rods 

 of quartz and felspar arranged perpendicular to the edges of felspar 

 crystals. In a rock from a quarry near Dullatur the quartz and 

 felspar rods are arranged radially and give an extinction-brush. 



The micropegmatite is nearly always riddled with long needles 

 of apatite. It also contains flecks of ' viriditic ' minerals, and is 

 frequently miarolitic. It has undoubtedly formed the channels 

 whereby weathering agents have percolated through the rock, as 

 evidenced by its own frequent decomposition and by the greater 

 alteration of the minerals contiguous to it.- As the rock becomes 

 finer in grain, the micropegmatite decreases in quantity and ceases to 

 be identifiable as such. It appears as a greyish or reddish crypto- 

 crystalline substance, riddled with apatite needles, filling up the 

 interspaces between the other constituents.^ In the basaltic contact- 

 rocks micropegmatite disappears and olivine very rarely occurs. 



' Teall, Brit. Petrog., 1888, p. 192. 



- Cf. HoUand, Q.J.G.S., 1897, vol. liii, p. 411. 



3 Cf. Teall, Q.J.G.S., 1884, vol. xl, p. 644. 



