G. W. Tyrrell — Alkaline Igxeoufi Rocks, West Scotland. 123 



The Benbeoch analysis is remarkable for low alumina, very high 

 magnesia, ferrous iron, and lime, combined with comparatively high 

 alkalies. It does not correspond exactly with any known analysis. 

 Its nearest affinity is the ultra-femicnepheline-basalt of Uvalde County, 

 Texas. Analyses II and III correspond fairly well with it in all 

 except the alkalies, which are much lower than in the Benbeoch rock. 

 This is accounted for by the absence of felspathoids in these rocks. 

 Compared with a typical theralite the kylite analysis shows a con- 

 siderable diminution in alumina and alkalies, but contains almost 

 three times as much magnesia for about the same silica percentage. 

 Kylite therefore appears to fall into place as the plutonic equivalent 

 of an ultra-femic nepheline-basalt. 



The quantitative mineral composition, as estimated by the Eosiwal 

 method, is given in column 1, Table V below, together with mineral 

 analyses of the principal variations of the rock. The labradorite in 

 each case includes a little orthoclase, perhaps 2 or 3 per cent. 



Table V. 



I. Kylite, Benbeoch, the dominant type, analysed by Dr. Dittricli. 



II. Kylite, a less femic facies, Benbraniachan. 



III. Kylite, a less femic facies, Craigmark. 



IV. Kylite-picrite, an ultra-basic facies, Chalmerston Hill, Benbeoch. 



It will be noted that all these rocks are dofemic, to use a convenient 

 term. Although the type represented bj^ I forms at least three- 

 quarters of the mass of Benbeoch, some distinct varieties, represented 

 by II, III, and IV, must be recorded. II and III are satellitic 

 outliers of the upper part of the Benbeoch sill, and are notably 

 richer in the felspathic constituents and poorer in olivine. IV, on 

 the other hand, forms an ultra-basic stratum at or a little below the 

 centre of the sill. This facies consists principally of large euhedral 

 to subhedral crystals of partially serpentinized olivine, forming 

 50-60 per cent of the rock. Large plates of titanaugite and fresh 

 plagioclase form the major part of the remainder. Both minerals 

 poikilitically enclose numerous small rounded olivines. The augite 

 and felspar are idiomorphic towards a turbid base, partly identifiable 

 as analcite and partly indeterminate, which encloses flakes of biotite 

 and large cross-fractured crystals of apatite. The felspar plates show 

 in unusual perfection the radiating fissures springing from enclosed 

 olivine. 



■^ This includes a little unidentified turbid matter. 



