78 Q. TV. Tyrrell — Alkaline Igneous Rocks, Wed Scotland. 



barkevicite 15 per cent. The felspar is also perfectly euhedral, and 

 very zonal. The determination of its composition is difficult because 

 of the analcitization it has undergone, but it is probably a labradorite 

 of the usual composition — Ab^ An^. It forms about 10 per cent of the 

 rock. Ilraenite passing over to leucoxene totals 3 per cent, and 

 apatite, wbich crowds the groundraass, about 2 per cent. Nepheline 

 occurs in the groundmass, but decomposition renders the recognition 

 difficult. Its presence is indicated by a streaky micaceous alteration 

 product, and by faint hexagonal or rectangular outlines occasionally 

 discernible in the groundmass. The veins penetrating the Lugar 

 picrite differ from tlie above in containing no titanaugite. The 

 proportion of barkevicite and apatite is greater. Rocks closely 

 resembling the above occur as schlieren, veins, and irregular patches 

 towards the top of the exposure of theralite at Barshaw House, near 

 Paisley. Recognizable nepheline is considerably more abundant, 

 however, than in the Lugar rock, and there are numerous small 

 crystals of aegirine. The Barshaw theralite here falls naturally into 

 its place as the melanocratic facies of lugarite. 



This rock stands apart from the recognized types of igneous rocks, 

 and can be fitted with no current name. It has therefore been called 

 lugarite after the type locality. As far as the analcite content goes it 

 resembles the heronite of Coleman.' Considering its composition and 

 its association with the Barshaw theralite, which closely resembles 

 the Madagascan bekinkinites, lugarite evidently has affinities with 

 the ijolites, and may be regarded as an ijolite in which the greater 

 part of the nepheline has been displaced by original analcite, and in 

 which barkevicite is a prominent constituent as well as augite. 



A rock with some resemblance to lugarite, but with considerably 

 more felspar, occurs at the upper contact of a great essexite-dolerite 

 sill at Howcommon Quarry, Craigie. 



5. Monohiquite. — A remarkable rock belonging to this group is 

 found in association with a picrite-teschenite complex at Carskeoch 

 Hill, near Patna. In hand-specimens it is black and compact, and 

 carries numerous huge phenocrysts of hornblende and biotite ranging 

 up to 3 inches in diameter. Microscopically it consists of a crowded 

 mass of idiomorphic granules of pale augite, with numerous ciystals 

 of magnetite and flakes of biotite. Olivine occurs abundantly as 

 small raicro-porphyi'itic crystals. All these are enveloped in a scanty 

 but pervading groundmass of analcite, which also occurs in numerous 

 irregular areas where its specific characters may be easily recognized. 

 It is partially carbonated, and the areas are surrounded by a thick 

 zone of augite granules many of which are aligned tangentially to the 

 margin. Tlie phenocrysts consist of the above-mentioned large horn- 

 blendes, which are of a red barkevicitic variety, and biotite, also a red 

 variety. A little red hornblende also occurs in the groundmass. The 

 rock contains numerous ocelli of various kinds, the commonest being 

 a simple radial aggregate of augite prisms. 



This rock is very similar to the porphyritic monchiquite of 

 Kilchattan, Colonsay.' It differs, however, in the presence of olivine 



^ Harker, Petrology for Students, 4th ed., 1908, p. 132. 



- Summary of Progress of Geological Survey for 1909, 1910, p. -52. 



