288 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XVIII. — The Trachytic and Allied Rocks of the Clyde Carbonifer- 
ous Lava - Plateaus. By G. W. Tyrrell, A.R.C.Sc., F.G.S., 
Lecturer in Mineralogy and Petrology, University of Glasgow. 
Communicated by Dr Horne, F.R.S. 
(MS. received May 20, 1916. Read June 19, 1916.) 
1. Introduction. 
The lavas of the Scottish Carboniferous are predominantly basaltic. 
True andesites and rhyolites are conspicuous by their absence, whilst 
trachytes and allied rocks are present in quite subordinate quantity. 
This association of basalt and trachyte is in accordance with the fact that 
the basalts have a slight alkaline cast, which is evidenced by an alkali 
content greater than that of the average basalt, and by the occasional 
presence of nepheline and analcite amongst their constituents. The 
transition from basalts to more acid types is accomplished, not by way 
of the andesites, but through the mugearites, volcanic rocks with chemical 
affinities to the essexites. All transitions from the more felspathic types 
of basalts, the Jedburgh and Markle types, can be traced through 
mugearites to trachytes or allied rocks. 
Hitherto the Scottish Carboniferous trachytes have been known chiefly 
from the East Lothian area, where they form a series of flows overlying 
the basalts of the Calciferous Sandstone, and also compose the rocks of the 
three massive plugs of Traprain Law, the Bass Rock, and North Berwick 
Law. The work of Hatch,* and that of the Geological Survey of 
Scotland, f have made these types well known. Barron, J and recently 
Lady MacRobert, § have described an interesting series of trachytic rocks 
of the same age from the Eildon Hills, near Melrose. The continuance of 
petrographical work upon the great Clyde plateau of Carboniferous basalts, 
so aptly named by Sir A. Geikie, has disclosed the fact that trachytes and 
allied rocks are widely distributed in the great masses of volcanic material 
stretching from Stirling, over the Clyde area, to South Bute and Campbel- 
town. These rocks occur generally as dykes, sills, and plugs, but rarely as 
distinct lava-flows ; and they will probably be found to bulk quite as 
* F. H. Hatch, Trans. Roy. Soc. Edinburgh , vol. xxxvii, 1892, pp. 115-126. 
t Geol. Surv. Mem. : Geology of East Lothian , 1910, pp. 127-133. 
t T. Barron, Geol. Mag. (iv), iii, 1896, pp. 371-378. 
§ Lady MacRoberl, Quart. Jour. Geol. Soc ., vol. lxx, 1914, pp. 303-315. 
