CHAP XII 
A RENIG ER UP TIONS—SCOTLAND 
191 
phyrites ” of the Old Eed Sandstone, and contain two forms of pyroxene — 
one rhombic, probably enstatite, and the other monoclinic angite. There are 
likewise considerable masses of intrusive rock, which are varieties of diabase 
or dolerite.’ 
iii. SCOTLAND 
From the centre of England we must in imagination transport our- 
selves into the Southern Uplands of Scotland, wliere a widely dis- 
tributed series of Silurian Amlcanie rocks has been preserved. It was, until 
recently, supposed that the Silurian system north of the Tweed contains no 
contemporaneously erupted volcanic rocks. Yet, as far back as the year 
1800, I pointed to the abundant existence of volcanic detritus in these 
strata throughout the southern counties as a probable indication of volcanic 
actmty at the time and in the area within which the strata were deposited.^ 
Some years latei’, when the microscope had been introduced as an aid to 
field-geology, I sliced some of the Silurian sediments of that region and 
found them, particularly certain shales and grits of Molfatdale, to contain a 
large admixture of perfectly fresh unworn felspar crystals, which I felt 
tolerably certain had been supplied by volcanic explo.sions. As no trace, 
however, had then been detected of an intercalated volcanic group in any 
part of the Silurian series of the south of Scotland, I used at that time to 
speculate on the possibility of the volcanic detritus having been wind-borne 
from the volcanoes of the Lake District. I had at that time 110 suspicion 
that its source was rather to be sought under my feet. The presence of 
volcanic rocks underneath the uplands of the south of Scotland would have 
been a welcome explanation of the frequent felspathic composition of inan^- 
of the Silurian greywackes and shales of that i-egion, and particularly the 
abundance of andesitic and felsitic fragments in them. 
It had been long known that the Scottish Silurian formations, besides 
having undergone extensive phcation, have also been injected by protrusions 
of igneous material of various kinds. The intrusive character of many of 
these is so obvious that a similar origin was attributed even to those bosses 
which could not be proved to be intrusive. Kecent work of the Geological 
Survey, however, and more especially the numerous and careful traverses of 
my friend and colleague Mr. Peach, have revealed the unlooked-for and 
important fact that a large number of these supposed intrusions are reallv 
portions of a volcanic group brought up on the crests of anticlinal folds, and 
laid bare by denudation. This group can be traced for at least 100 miles 
from north-east to south-west over a belt of country sometimes oO miles 
broad. Its original limits cannot be ascertained, but they obviousl}’ exceeded 
those within which the rocks can now be seen. Nevertheless the present 
boundaries embrace an area of nearly 2000 square miles. Tins PahTozoic 
volcanic region is tlms one of the most extensive in the British Isles. 
1 See Mr. \V. W. AVatts on the Igneous and Associated Rook.'i of the Breidden Hills, Quart. 
Journ. (ir.ol. Sue. vol. xli. (1885), ji. 532. 
^ Trann. Roy. Soc. Edin, xxii. (1860), p. 636. 
