362 G. W. Tyrrell — Intrusions of Kilsyt?i-Croy District. 



The annexed analyses of similar British rocks illustrate the 

 remarkable uniformity of composition prevailing in this type. All 

 are diabasic rocks with quartz or micropegmatite. The only point 

 of difference in the Kilsyth rock is its higher content of alumina and 

 its lower content of alkalis as compared with the rocks of the Whin 

 Sill and St. Davids. 



In Table II, cols, i-iv below, is given the mineral composition 

 (norm) of the above rocks, calculated according to the method of 

 Cross, Iddings, Pirsson, and "Washington.^ The augite is assumed to 

 have the theoretical composition Ca 0, (Mg . Fe) 0, 2 Si Oo, and the 

 hypersthene Mg 0, Fe 0, 2 Si Og. 



A remarkable point about these calculated mineral compositions is 

 the very large proportion of orthorhombic pyroxene they show. This 

 is especially marked in the Kilsyth rock. The actual proportion of 

 this mineral, however, is usually small and much lower than that of 

 monoclinic pyroxene. In the Kilsyth rock it is very small indeed. 

 The inference is, then, that the monoclinic pyroxene contains 

 a considerable admixture of the hypersthene molecule. This has 

 been directly proved by analysis in the case of the Whin Sill rock, 

 the similar 'augite-diorite' of Southern India, and the St. Davids rock. 



The mineral composition of these rocks shows much greater 

 uniformity if quartz and orthoclase (= micropegmatite) be taken 

 together, also albite and anorthite ( = plagioclase), augite and hyper- 

 sthene (= pyroxenes), and magnetite and ilmenite (= iron-ores), as 

 shown in Table II, cols, v-viii. 



VII. Affinities and Relationships of the Kilsyth-Ceot Rocks. 



The rocks described in this paper are identical in chemical com- 

 position and down to the minutest microscopical details with the Whin 

 Sill, as described by Dr. Teall.^ The latter, indeed, is the most 

 southerly member of the great group of late or post- Carboniferous 

 dykes and sills of South Scotland and North England. They are 



Quant. Class, of Igneous Socks, 1903, p. 186 et seq. 



2 Op. cit., p. 28. 



