CHAP. XXV 
D YKES AND SILLS OF THE PLA TEA UX 
407 
nected with the Tertiary basalt-plateaux, like which they may have had a 
close relation to the actual l)uilding up of the successive sheets ot andesite, 
trachyte and basalt tliat were erupted at the surface. They are j)articulaily 
well developed in the Clyde plateau, where by extensive denudation they have 
been admirably exposed. I would especially refer to those that traverse the 
tract of red sandstones which underlie the volcanic series along the flanks of the 
great escarpments from Fintry to Strathblane and Dumbarton, and between 
Gourock and Ardrossan. These dykes have been dissected by the sea along 
both sides of the estuary of the Clyde and in the islands ot Gumbrae. In these 
islands and in Bute they have recently been mapped in great detail for the 
Geological Survey by my colleague, Mr. W. Gunn, who has supplied me with 
notes of his observations on the subject, from which the following summary 
is compiled. 
“ There are at least four distinct groups of intrusive rocks in the Greater 
Gumbrae. The oldest of these is trachytic in character, and occurs 
both as dykes and sheets, which run generally in the same E.N.E. direction. 
The rock is usually pinkish in colour, sometimes grey or purplish. A 
specimen from the dyke of the Hawk’s Xest, north of harlaud Point, 
analyzed by Mr. Teall, was found to contain 11 per cent of alkalies, 
principally potash, while the percentages of lime and iron were very low. 
Sometimes these rocks are fine in grain with only a few porphyritic ortho- 
clase crystals, though numerous small crystals of this mineral are found 
with the aid of the microscope. These red trachyte dykes are almost con- 
fined to the Upper Old Bed Sandstone, rarely entering the overlying white 
Calciferous Sandstones, and never invading the plateau-lavas. They are 
therefore probably of early Carboniferous age. 
“ The next gi’oup follows the same general direction, but clearly traverses 
the trachytes, and must therefore be of later date. The dykes of this group 
are the most numerous of the whole, the greater part of the island being 
intersected by them. In the north-east corner about 40 of them may be 
counted in half a mile of coast-line, some being of large size. All of them 
which can be clearly made out arc porphyritic olivine - basalts of the 
type of the Lion’s Haunch at Arthur’s Seat. They are generally grey in 
colour and finer at the edges than in the centre, which is often coarsely 
porphyritic and amygdaloidal. Olivine seems always characteristic, but has 
often been replaced by hsematite or calcite. In Bute a good many dykes 
have been mapped to the north of Kilchattan Bay resembling this basalt 
series of Gumbrae, and running in the same direction. But they appear to 
be all porphyritic andesites. The second group of dykes, though it cuts 
the first and is thus proved to be later in date, is nevertheless confined 
within the same stratigraphical limits. It may thus belong nearly to the 
same period of intrusion. 
“ The dykes of the third group are dolerites without olivine, and follow 
on the whole an east and west direction. Tliey cut both of the two fore- 
going sets of dykes, and likewise the lavas of the plateau. They must thus 
belong to a far later period of intrusion. They may be connected with 
