1915-16.] Trachytes of Clyde Carboniferous Lava-Plateaus. 289 
largely in the Clyde plateau, especially in its south-western portion, as do 
similar rocks in the East Lothian area, although it must be remembered 
that they are always subordinate to the basaltic types. In the present 
paper it is desired to systematise our knowledge of the trachytes and 
allied rocks of the Clyde plateau, and also to record and describe some 
hitherto unrecognised occurrences. 
2. Classification. 
The rocks may be classified as follows : — 
(a) Albite-bostonite, albite-trachyte, and albite-keratophyre. 
The essential character of this group is the predominance of a felspar 
near to albite. Many of the rocks also contain phenocrysts of sanidine, 
and have a little quartz in the groundmass. The terms bostonite, trachyte, 
and keratophyre are here used with textural significations for rocks 
which have essentially the same chemical and mineral composition. The 
bostonites are of comparatively coarse but equidimensional texture. The 
constituent felspars generally have a more or less perfect fluxional 
arrangement. The trachytes are of much finer grain, and their groundmass 
is composed of minute felspar laths with a good fluxional or trachytic 
texture. In the keratophyres the groundmass is dense and felsitic, and 
its constituents are devoid of fluxional arrangement. These terms are 
used regardless of whether the rocks are of intrusive or extrusive origin. 
This group contains the majority of the rocks described in this paper. 
They occur abundantly in the Great Cumbrae island, South Bute, the 
Misty Law district and Neilston Pad (Renfrewshire), and in the Campsie 
and Kilsyth Hills (Stirlingshire). 
(b) Bostonite , trachyte, and keratophyre. 
In this group orthoclase felspar approximately equals the albite in 
quantity. The orthoclase molecule appears as phenocrysts of sanidine, and 
is also prominent in the groundmass. The textures of these rocks cor- 
respond generally with those of the above group. These types are not so 
abundant as the foregoing, and occur only in the Great Cumbrae and in the 
Misty Law district. 
(c) Quartz-keratophyre and Felsite. 
These rocks, while corresponding generally with those of the first two 
groups in mineral composition, are considerably richer in quartz. Rocks 
with the bostonitic or trachytic textures have not been found — a fact to be 
correlated, perhaps, with the change in chemical composition. In the 
quartz-keratophyres the alkali-felspars can be identified with fair certainty. 
VOL. xxxvi. 19 
