G. W. Tyrrell — Intrusions of Kilsyth-Croy District. 361 



attributed to an increase in the proportion of the femic constituents. 

 At the actual margin spherulitic and tachylytic structures sometimes 

 occur, and where in contact with carbonaceous strata the rock 

 suffers the endomorphic metamorphism resulting in the production 

 of ' white trap '. 



A slide from a specimen taken from the lower margin of the great 

 Sauchie sill near Stirling shows sparse but uniformly scattered felspar 

 microlites in a greyish cryptocrystalline ground-mass, with numerous 

 small grating-like skeletal crystals of ilmenite. There are interspersed 

 numerous small microporphyritic groups of fresh lathy plagioclase. 

 Here and there are small corroded xenocrysts of quartz. 



YI. The Chemical Composition. 



Mr. D. P. Macdonald, M.A., B.Sc, Baxter Demonstrator in Geology 

 at Glasgow University, has kindly made me a complete analysis of the 

 normal type from the Auchinstarry Quarry, Kilsyth. 



The result is given in Table I, col. i, below — 



TABLE I. 



I. Diabase, with micropegmatite (granophyric diabase), Auchiastan-y, Kilsyth. 

 II. Diabase, TMiin Sill, Crags near Roman Station of Bourgovicus^ Northumberland. ' 

 Diabase, dark-coloured modification (somewhat weathered), Kettlestoua Quarry, 



Bathgate Hills.^ 

 Quartz ' Gabbro (intermediate type), east flank, Carn Llidi, 10 yards from 

 margin of sill, St. Davids.* 



Ill 

 lY 



' Teall, Q.J.G.S., 1884, vol. xl, p. 654. 



2 Falconer, T.R.S.E., 1905-6, vol. xlv, pt. i, p. 147. 



* Ebden, Q.J.G.S., 1908, vol. Ixiv, p. 281. 



