306 G. W. Tyrrell— Intrusions of Kikyth-Croy District. 



The sp. gr. of the normal variety varies from 2"90 to 2-93, whilst 

 a coarse rock with large areas of pink micropegmatite from the 

 Dumback Quarry at Croy has a sp. gr. of 2'82. 



Microscopical Characters. 



In thin section the normal rock is found to consist of a framework 

 of interlocking crystals of plagioclase, augite, and iron-ores, with 

 micropegmatite occupying the interspaces and generally playing the 

 part of ground-mass. Hypersthene occurs sporadically with the augite. 

 Among the secondary minerals are hornblende, chlorite, biotite, 

 serpentine, caleite, and a green pleochroic bastite. 



The felspars give long columnar sections, with the edges in the 

 prism zone well defined, but terminations rarely recognizable. 

 Squarish untwinned sections, parallel to 010, are rather abundant; 

 Carlsbad and albite twinning is common, but the pericline is distinctly 

 rare. The maximum extinction angle in albite-twinned sections 

 giving symmetrical extinction is about 28°, which corresponds to a 

 composition Abg Ang, and indicates a labradorite of medium basicity. 



Chemical zoning is very prominent. Whilst the interior of a crystal 

 may give the maximum extinction angle, the border may give straight 

 extinction, thus indicating a variation in composition from labradorite 

 to oligoclase. This zoning is most pronounced in the square 

 untwinned sections parallel to 010. This is due to the greater range 

 of the extinction angles for the different varieties of felspar in sections 

 parallel to 010 than for sections in other directions.^ In some 

 specimens from the Milngavie intrusions the felspars are zoned by 

 inclusions. 



The alteration of these felspars is a simple kaolinization in the more 

 acid peripheries, but granules of caleite occur in the more basic kernels. 

 Flecks of viriditic material are often to be found inside the felspar, 

 indicating some degree of migration of the more mobile alteration 

 products from the ferromagnesian minerals. 



The augite is pale-brown, yellow, or almost colourless in the Kilsyth- 

 Croy rocks, but in the Milngavie specimens there is often a faintly 

 pleochroic purple tinge along the borders. A narrow twin lamellation 

 confined to the centre of the crystal is prominent. In some of the 

 coarser gabbroid varieties a basal striation occurs in a sahlite-like 

 augite. In these rocks, also, the augite tends to occur in a long blady 

 form elongated along the ' C ' axis. 



In some sections it appears to take on to some extent the pleochroism 

 and polarization colours of hypersthene, but not the straight extinction. 

 The chemical analysis (infra) shows that the hypersthene molecule 

 must be present to a large extent. This corresponds with the 

 observations of Teall^ and Elsden^ on the augites of similar rocks 

 from Northumberland and Pembrokeshire respectively. 



The alteration of the augite is a process of considerable interest. 

 The most common type is a peripheral change to brown hornblende. 



1 See Iddings, Rock Minerals, 1906, p. 229. 



2 a.J.G.S., 1884, vol. xl, p. 648. 



3 Q.J.G.S., 1908, vol. Ixiv, p. 286. 



