200 
THE SILURIAN VOLCANOES 
BOOK IV 
detected by Messrs. Peach and Horne and determined by Mr. Teall. They 
are associated with the Bala limestone, which in some of its conglomeratic 
bands contains pebbles of fclsite. 
The intrusive rocks which accompany the Lower Silurian volcanic series 
of the south of Scotland are best displayed in the south-west of Ayrshire, 
between Girvan and Ballantrae, where they appear to be on the whole 
later than at least the great mass of the interstratified lavas and 
tuffs. The most abundant rocks and the earliest to' be injected are 
complex basic masses which include serpentine, olivine-enstatite rock, 
troctolite, gabbro and other compounds, all which may be different modifica- 
tions of the same original basic magma. They do not show a finer texture 
where they respectively' meet, nor any other symptom of having been subse- 
quently intruded into each other, though they do exhibit such structures 
along their lines of contact with the surrounding rocks, into which they 
are intrusive. These more basic masses have subsequently been invaded by 
irregular bosses and dyke-like protrusions, which, when small, are fine- 
grained dolerites, but when in larger bodies take the form of gabbro, 
sometimes exhibiting a mineral banding and foliated structure. These 
banded varieties much resemble the banded Tertiary gabbros of Skye and 
some parts of the Lewisian gneiss. 
At the Byne Hill, near Girvan, a large intrusive boss or ridge displays 
on its outer margin a fine-grained texture, where it comes in contact with 
the serpentine. Further inwards it becomes a fine dolerite, passing into gabbro 
and increasing in coarseness of grain as well as in acidity of composition, 
through stages of what in the field would be called diorite and quartz-diorite, 
into a central granitic rock, whereof milky or blue quartz forms the prominent 
constituent. The intrusive rocks of this district have generally been ijijected 
parallel to the stratification-planes, and take on the whole the form of sills. 
Some time after the close of the volcanic episode in the Silurian period 
of the south of Scotland, the rocks were hKially subjected to considerable 
disturbance and elevation, whereby parts of the volcanic series were exposed 
to extensive denudation. Hence the overlying unconformable Caradoc 
conglomerates are in some places largely made up of the detritus of the 
volcanic rocks. It is interesting to find this evidence of waste during 
the very next stage of the Silurian period, for it affords good evidence 
that the extensive sheets of intrusive material could not have had any 
large amount of overlying strata resting upon them at the time of their 
injection. Pieces of these intrusive rocks, such as the serpentine, occur 
abundantly in the Caradoc conglomerates, some of which indeed are 
almost wholly composed of their detritus. Probably the total thickness 
of the overlying cover of rock under which the sills were injected did 
not amount to as much as 200 or 300 feet. Yet we see that among 
the sills were coarse gabbros and granitoid rocks. We may therefore infer 
that for the injection of such intrusive masses, great depth and enormous 
superincumbent pressure are possibly not always necessary. 
