290 Proceedings of the Royal Society of Edinburgh. [Sess. 
The rocks have been called felsites when the texture becomes excessively 
dense and cryptocrystalline, and when there is no possibility, apart from 
chemical analysis, of determining the nature of the principal mineral 
constituents. 
These rocks are not so abundant as those of the first group. An 
excellent quartz-keratophyre occurs in the great Craigmushat sill at 
Gourock (Renfrewshire), and others in the Great Cumbrae and the Misty 
Law district. Felsites occur in the Great Cumbrae and in the Campsie 
Fells. 
(d) Phonolite. 
Trachytic rocks carrying nepheline are at present definitely known 
from only one locality, Fintry (Stirlingshire), in the west of Scotland. A 
new locality will probably be found in North Ayrshire, as heaps of road- 
metal consisting of nepheline-bearing phonolitic trachyte have been 
noticed near Darvel and Galston. 
3. Petrography. 
(a) Albite-bostonite, albite-trachyte, and albite-keratophyre . 
In hand specimens these are compact, grey, yellowish, or dark-coloured 
rocks, exhibiting small flesh-coloured phenocrysts of felspar. They 
frequently have a platy structure, and in the more trachytic varieties 
show a resinous lustre on freshly broken surfaces, and have a sonorous 
ring under the hammer. 
In thin section they show phenocrysts of sanidine, and occasionally 
albite and anorthoclase, in a groundmass which consists predominantly 
of laths of a felspar near albite in composition, with subordinate ortho- 
clase. There is generally a diffused yellowish stain of limonite over 
phenocrysts and groundmass alike, and a widespread dissemination of 
minute chlorite scales. Sanidine forms by far the most abundant set of 
phenocrysts, and frequently occurs alone. It forms large, elongated,, 
rectangular, simply-twinned crystals, orientated in the direction of flow, 
and frequently occurring in sporadic groups of two or three. Albite and 
anorthoclase occur as phenocrysts much less often than sanidine, and 
generally form much smaller crystals. Albite has only been noticed in 
nodules of albite-keratophyre enclosed in the bostonite dyke north of 
Keppel Pier, Great Cumbrae, in an albite-trachyte from the March Burn,. 
Kilsyth Hills, and in an albite-bostonite from South Bute. The felspar 
identified as anorthoclase occurs in small crystals which have a peculiar 
mottled appearance in polarised light and frequently show a minute 
