1915-16.] Trachytes of Clyde Carboniferous Lava-Plateaus. 293 
increase in the amount of potash felspar until it equals or exceeds that of 
the soda felspars. Augmentation of the potash felspar may take place by 
an increase in the number of sanidine phenocrysts, or by an increase in the 
amount of orthoclase in the groundmass, or in both ways simultaneously. 
In the chemical analyses of the albite-bostonites there is a decided excess 
of soda over potash, whilst the bostonites proper show approximate 
equality between these constituents. 
It is unnecessary to give a full petrographical description of these rocks, 
as it would merely duplicate that given in the previous section, with the 
single qualification that orthoclase is relatively more abundant both among 
the phenocrysts and as a constituent of the groundmass. 
The rock of the Barbay sill in the centre of Great Cumbrae island is 
a typical bostonite of this group. Other dykes and sills in the Cumbrae 
may also be referred to this group, notably one from the Hawk’s Nest, 
north of Farland Point, in which Dr J. J. H. Teall found on analysis 
11 per cent, of alkalies, principally potash, whilst the percentages of lime 
and iron were very low.* Mr J. V. Harrison found 8‘75 per cent, of 
alkalies in the Barbay sill, divided up almost equally between soda and 
potash (see Table I, 5). 
Trachytes belonging to this group occur in the March Burn, Kilsyth 
Hills, and in the Misty Law area. A dyke from the latter district shows 
clusters of sanidine and anorthoclase (?) phenocrysts in a very dense 
groundmass consisting of fluxionally arranged felspars which are mainly 
orthoclase. The only other constituents are diffused areas of chlorite and 
hsematite, with a little magnetite. These types pass over gradually into 
those in which the groundmass becomes dense, felsitic, and devoid of 
fluxional arrangement. A keratophyre dyke from the Misty Law area 
illustrates this group. It consists of numerous microphenocrysts of 
anorthoclase in a dense quartzo-felspathic groundmass containing a few 
prisms of green soda-pyroxene. Similar rocks are mentioned as occurring 
in the Meikle Bin area.]- The rocks of this group may perhaps be 
correlated with the porphyritic and non-porphyritic trachytes of East 
Lothian, J and with the sanidine trachytes of the Eildon Hills. § 
(c) Quartz-Jeer at ophy re and Felsite. 
This group is more highly siliceous than either of the foregoing. 
The mineralogical expression of this chemical fact is the abundance of 
* Mem. Geol. Surv. : North Arran, South Bute, and the Gumbraes, 1903, p. 62. 
t Mem. Geol. Surv. : Geol. of Glasgow District, 1911, p. 145. 
f Mem. Geol. Surv. : Geol. of East Lothian, 1910, p. 132. 
§ Lady MacKobert, Q.J.G.S., vol. lxx, 1914, p. 305. 
