292 
VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
felsitic sill foniis a consj^icuous feature along the north side of the chain, 
and there are probaLly others which have not yet been separated from 
the felsitic tuffs and orthophyres which they so much resemble. 
Perhaps the most remarkable acid 
sills in the Old Eed Sandstone of 
Britain are those which occur at the 
extreme northern end of the region 
among the volcanic phenomena of the 
Shetland Isles (Figs. 71, 72). The 
largest of them, consisting mainly of 
granite and felsite, is believed to reach 
a length of 2 0 and a breadth of from 
three to four miles.^ 
A group of sills composed of a 
bright red quartz-porphyry has been 
traced along the southern flanks of 
the Highlands for upwards of 18 
miles.- This rock, already referred to 
as the “ Lintrathen porphyry,” lies 
chiefly among the conglomerates and 
sandstones, but also intersects the lavas, 
and may be later than the Old Ked 
Sandstone (p. 277). An extension of 
it is found even on the north side of 
the boundary fault, cutting the ande- 
sites which there lie unconformably 
on the schists. 
Examples, however, occur of sills 
much less acid in composition. In 
the Dundee district, for instance, the 
intrusive sheets are andesites and 
diabases. They send veins into and 
bake the sandstones among which they 
have been intruded, and are sometimes 
full of fragments of such indurated 
sandstone, as may be well seen on 
the northern shore of the Firth of 
Tay, west of Dundee. 
A conspicuous characteristic of 
most of the volcanic tracts of the 
Lower Old Red Sandstone is the com- 
parative scarcity of contemporaneous 
dykes. In the band of acid sills 
between Muirkirk and the Clyde, a considerable number (fl‘ dyke.s 
^ Messrs. B. It. Peach and J. Home, Trans. Roy. Soc. Edin. xxxii. (1884), p. 359. 
- See Sheet 56 of the Geological Survey of Scotland. 
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