of the Clyde Area. 309 



developed in approximately equal quantity), and perpatic (ground- 

 mass overwhelmingly predominant over phenocrysts). 1 



Table I. 



Specific gravity . 2-706 2-680 

 The composition of the glass may be calculated from the chemical 

 analysis (Table II, 1) by deducting the mineral constituents in the 

 proportions shown in Table I. After deducting 7 per cent of 

 anorthite (AbiAn 9 ) and 31 per cent of labradorite (AbxAn^, and 

 assigning all the Ti O2 and Ee 2 3 to ilmenite and magnetite 

 respectively, the rest of the ferrous iron and lime with magnesia 

 and the proper quota of silica form just 20 per cent of pyroxene, 

 thus affording good evidence of the accuracy of both mineral and 

 chemical analyses. All the mineral constituents have now been 

 deducted ; and as the ilmenite and magnetite may be held to 

 represent the dark globulites in the glass, the remaining potash, 

 soda, alumina, and silica must be present in the colourless glass, 

 which must have the approximate composition Si O2, 80"4 ; A1 2 3 ,- 

 9-9; Na 2 0, 3-7; K 2 0, 6-0. There is a small defect of Al 2 3 as 

 compared with the amount required to make orthoclase and albite 

 molecules of all the potash and soda. It is possible, therefore, that 

 there is a slight defect of Al 2 O s in the analysis of the rock. From 

 these results it may be concluded with fair probability that, apart 

 from the dark globulites, there is no iron, magnesia, or lime in the 

 glass, and that it consists entirely of orthoclase, albite, and free 

 silica molecules. 



Chemical Composition. 

 The chemical composition of the Cumbrae dyke and of the similar 

 dyke of Eskdalemuir, is set out in Table II, 1, 2. Compared with 

 average andesite (II, 5) and average hypersthene-andesite (II, 6), 

 they have the same amount of silica and alkalies, less alumina, but 

 higher ferrous iron and lime. The Cumbrae types are therefore slightly 

 more femic than the average andesites, but have a somewhat more 

 sodic plagioclase felspar, as is well shown by the norms (Table III, 

 1, 2, 5, 6). 



The comparison between the two cumbraites is hindered by the 

 non-determination of Ti O2 in the Eskdale rock ; but the smaller 

 A1 2 3 and lime, and the higher ferrous iron and magnesia, of the 

 latter, cause a reversal of the relative proportions of diopside and 

 hypersthene in the norms of the two rocks (Table III, 1, 2). The 

 Eskdale rock is slightly more femic and less quartzose than the 

 Cumbrae rock, as is shown by the dashes following the symbols for 

 class and order in the former. 



1 Iddings, Igneous Rocks, vol. i, pp. 187, 199, 1909. 



