1915-16.] Trachytes of Clyde Carboniferous Lava-Plateaus. 295 
different from the quartz-keratophyres in chemical composition. They 
differ from them by containing a large proportion of dense crypto- 
crystalline matter, and in their distinctively felsitic texture. They are 
usually rich in quartz. 
A group of dykes north of Downcraig Ferry, on the east coast of the 
Great Cumbrae, well illustrates the characters of the felsites. One of 
these dykes provides a most striking rock in hand specimens, as it is 
banded in broad stripes of yellow and purple. The groundmass of these 
rocks is usually quartzo-felspathic, and may either be minutely crystalline 
or with a good deal of crypto-crystalline material in it. Occasionally 
hair-like microlites apparently of some iron-ore are numerous in the 
groundmass. Some of the dykes are non-porphyritic ; others have felspar 
phenocrysts which have been entirely replaced by chalcedonic silica, 
without loss of crystalline form. They are all stained yellow and red with 
limonite or haematite, but the only indications of original ferro-magnesian 
constituents are occasional pseudomorphs after biotite in magnetite. These 
rocks are probably identical with a felsite from the Meikle Bin vknt, of 
which an analysis is given later (Table I, 10). 
(d) Phonolites. 
The Fintry occurrence, which, with one unlocated exception, is the 
only phonolitic rock yet discovered in the Clyde area, is fully described 
in the Glasgow Memoir (p. 144). Its clear mineralogical and chemical 
resemblance to the rock of Traprain Law is there illustrated and com- 
mented upon. 
An excellent phonolite has also been found by Miss A. T. Neilson in 
heaps of road-metal in the Galston district of Ayrshire. This rock should 
be called a phonolitic-trachyte, perhaps, rather than nepheline-phonolite, as 
nepheline occurs only in insignificant quantity. In thin section the rock 
has a tr.achytic groundmass consisting of flow-orientated orthoclase laths 
interspersed with grains of segirine-augite and magnetite. In this are 
numerous phenocrysts of orthoclase in which euhedral and somewhat 
altered nepheline is embedded or intergrown. Microphenocrysts of segirine- 
augite are numerous ; and olivine, invariably altered to iddingsite, also 
occurs in the same way but in smaller amount. This rock is much more 
mafic than the nepheline-phonolite of Fintry, and closely resembles the 
olivine-augite-trachyte or orthophyre of the Eildon Hills, described by 
Lady MacRobert.* 
* Op. cit., p. 309. 
