of the Clyde Area. 351 



moderately basic plagioclase, magnetite, and of dark' interstitial 

 glass". Teall describes the microscopical character of the Brunton 

 dyke as follows: "long, narrow, lath-shaped felspars, irregular 

 crystalline grains and plates of a nearly colourless pyroxene, and 

 a small quantity of nearly opaque interstitial matter." The rock is 

 figured in pi. xii, fig. 6, of his paper. Judging from the figures 

 given by Harker & Teall, and from thin sections of Brunton types 

 from the North of England dykes in the collection of the Geological 

 Department of Glasgow University, the felspar and augite are 

 approximately equally developed, with perhaps a slight excess of 

 the pyroxene, and the amount of glass is only slightly subordinate 

 to either of the crystalline constituents. The Tynemouth dyke and 

 several of the Ayrshire examples carry large phenocrysts of bytownite 

 or anorthite of exactly the same characters as in cumbraite or 

 inninmorite. Magnetite may or may not be present, and the texture 

 is typically intersertal or tholeiitic. The type is clearly more basic 

 than leidleite, inninmorite, or cumbraite, a fact reflected in the 

 diminished quantity of glass. Norms calculated from the analyses 

 of the Brunton type published by the Geological Survey show no 

 quartz and 5 per cent of orthoclase in the 1914 analysis, and 

 4"2 per cent of quartz and 5 - 5 per cent of orthoclase in the 1915 

 analysis. Both rocks fall into the subrang camptonose (III, 5. 3. 4), 

 showing at once that they are richer in the femic minerals than 

 cumbraite, etc., and that there is only a negligible amount, if any, 

 of excess silica. 



FlG. 2a. — Tholeiite (Brunton type), N.N.W. dyke, Stairaird, Biver Ayr, near 

 Mauchline, Ayrshire. x 20. Large phenocryst of bytownite- 

 anorthite ; groundmass : grains of augite, and laths of labradorite, in 

 base of dark glass. 

 ,, 2b. — Olivine-tholeiite (Corrie type), N.N.W. dyke, Birchpoint, Corrie, 

 Arran. x 20. Corroded phenocrysts of bytownite-anorthite ; ground- 

 mass : laths of labradorite, grains of augite, olivine, and magnetite, in 

 scanty base of dark glass. 



