CHAP. XVIII 
SILLS AND DYKES 
291 
connected with superficial volcanic discharges of which no remains have 
survived, they seem to indicate the wide extent and remarkable vigour of 
the subterranean igneous action of this geological period. 
Viewed as a whole, the materials which now occupy the vents of the 
volcanic chains in the Lower Old Red Sandstone of the tritish Isles are 
more acid than the lavas erupted at the surface. In the Pentland district, 
indeed, and in some other areas this acid material was ejected at intervals in 
abundant discharges of dust and lapilli and in outflows of felsitic lavas, 
while between these successive discharges copious streams of diabasic and 
Fig. 70. — Seetiou acro-ss the three Dirriiigtou Laws, Berwickshire. 
a Upper Silurian strata ; ?>, Xeclcs probably of Lower Old Red Sandstone age ; c, Upper Old Red Sandstone lying 
’ unconfunnably both on a and h. 
andesitic lavas, either from the same or from some closely-adjoining vent, 
were poured out. Throughout the whole region, however, as a closing phase 
of the volcanic history, the acid magma rose after the outpouring of the more 
basic lavas and filled such chimneys of the volcanoes as were not already 
blocked with agglomerate. It was probably after these pipes were plugged 
that the final efforts of volcanic energy were expended in the protrusion of 
the acid material as sills between the bedding-planes of the surrounding 
rocks, and as dykes and veins in and around the vents. 
iii. SILLS AND DYKES 
Nowhere throughout the volcanic tracts of the Lower Old Red Sandstone 
is there any such development of sills as may he seen beneath the Silurian 
volcanic sheets of North Wales. Those which occur are most abundant in 
the Lanarkshire district, to the north-west and south-west of iinto, <ind in 
the south of Ayrshire. IToni the village of Muirkirk to the gorge of the 
Clyde, below the Falls, the Upper Silurian and Lower Old Red Sandstone 
strata are traversed by numerous intrusive sheets of pink and yellow felsite, 
quartz-porphyry, minette, lamprophyre and allied I’ocks, which are no doubt 
to be regarded as part of the volcanic phenomena with which we are here 
concerned. In the south of Ayrshire, between the villages of Ualmellington 
and Barr, there is a copious development of similar sills, especially along one 
or more horizons near the base of the Old Red Sandstone. Garleffin Fell, 
Glenalla Fell, Turgeny and other heights are conspicuous prominences 
formed of these rocks ; above the sills lie thick conglomerates and sandstones 
on which the great andesite-sheets rest. 
In the Pentland Hills, as will be described in Chapter xx., a massive 
