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bostonites, and have a good fluxional or trachytic texture. The ground- 
mass of the keratophyres, on the other hand, whilst felspathic, is very dense, 
and the constituents are unorientated, giving a felsitic aspect to the rock. 
Albite-bostonite is by far the most abundant variety. It forms many 
of the dykes, sills, and bosses of the Great Cumbrae, and several lavas 
intercalated with the Calciferous Sandstone flows of South Bute. It also 
appears in the Ayrshire mainland in the Misty Law district, forms the plug 
of Neilston Pad (Renfrewshire) piercing Calciferous Sandstone basalt lavas, 
and dykes in the March Burn district of the Kilsyth Hills. An alkali 
determination, by Mr J. V. Harrison, of a dyke contiguous to a quartz- 
dolerite dyke in the March Burn gave soda 5 85 per cent., and potash 
2*71 per cent. The biotite- trachyte dykes of the Meikle Bin, Campsie 
Fells, probably belong here, as also do the albite-keratophyres of the same 
district mentioned in the Glasgow Memoir (p. 145). 
The Great Cumbrae rocks have been briefly described as trachyte and 
bostonite (?).* They were regarded as being prior to the Calciferous 
Sandstone lavas of South Bute and the Little Cumbrae, and consequently 
as the earliest of the Carboniferous igneous series. The evidence from 
other parts of the Clyde plateau, however, and also certain evidence 
provided by the Cumbrae trachytes themselves, point clearly to these 
rocks having been one of the later eruptives of the Lower Carboniferous 
suite.f They appear to be closely associated with the great tuff vents of 
the Meikle Bin type (Meikle Bin in the Campsie Fells, and Misty Law in 
the Renfrewshire Hills). J The Great Cumbrae area of trachytic dykes lies 
ten miles south-west of Misty Law, and may be related to that great vent ; 
but it is perhaps more probable that they are related to a vent of the 
Meikle Bin type now buried beneath the waters of the Firth of Clyde. 
Numerous sills, bosses, and dykes of trachytic rocks are to be found along 
the shore of the Firth between Ardrossan and Fairlie, but these have not 
yet received detailed attention. § 
(b) Bostonite , trachyte , and Iceratophyre. 
The albite-bostonites, albite-trachytes, and albite-keratophyres pass 
insensibly into true bostonites, trachytes, and keratophyres by a gradual 
* Mem. Geol. Surv. : North Arran , South Bute , and the Cumbraes, 1903, p. 177. 
f Mem. Geol. Surv. : Geol. of Glasgow District , 1911, p. 110. 
I Ibid., p. 104, 109. 
§ Summ. Prog. Geol. Surv. for 1913 (1914), p. 58. Since the above was written, 
Mr G. Y. Wilson, of the Scottish Geological Survey, has read a paper to the Glasgow 
Geological Society (March 9, 1916) on the vents of North Ayrshire, in which some of 
these trachytic rocks are described in detail. See “Preliminary Notes on Volcanic Necks 
jn N.W. Ayrshire,” Trans. Geol. Soc. Glasgow , vol. xvi, pt. 1 (1915-16), 1916, pp. 86-99. 
