OF THE TRAP-ROCKS OF SCOTLAND. 637 
abundant suite of fossils, lately detected by the Geological Survey, has shown to 
be on the parallel of the Ludlow Rock. We cannot, of course, from such evi 
dence, decide upon the actual eruption of volcanic matter during the Lower 
Silurian period in Scotland, though, considering the great volcanic activity at 
that time manifested in North Wales, such eruption seems highly probable. 
Future labours in the great Silurian region of the south of Scotland will doubtless 
clear up this interesting question. In the meantime we have evidence that, 
whether erupted then or previously, there were at that time felstones in suffi- 
cient abundance to contribute, by their drifted debris, a marked feature to many 
of the grits and conglomerates throughout a large part of the Silurian district of 
South Scotland, from Berwickshire to the borders of Lanark. 
Traps of Upper Silurian (?) and Lower or Middle Old Red Sandstone. 
The junction of Lower and Upper Silurian strata in Southern Scotland has still 
to be traced. That it will be found to exist as an unconformity seems probable, 
though it is impossible at present to conjecture whether or not the unconformity 
is a violent one, and what amount of denudation the older series underwent 
before the deposition of the newer. If it be shown that the amount of the dis- 
turbance prior to the Upper Silurian was great, then we may perhaps refer to 
this period of movement part at least of the igneous rocks by which the lower 
Silurian region is intersected. At the same time, it must be borne in mind that 
felspathic dykes occur abundantly in the Upper Silurian and lower Old Red Sand- 
stone series of Lanark and Ayrshire; and as these strata are often much tilted 
and broken, we shall probably not err in referring the igneous protrusions to the 
period of disturbance. It is quite possible, therefore, that the igneous rocks 
which so diversify the otherwise monotonous geology of Southern Scotland may 
be of two ages, one during the uptilting of the Lower Silurian, the other during 
that of the Upper Silurian and Lower Old Red Sandstone. At present I can deter- 
mine only the latter series, and I shall present here the proofs on which this 
determination rests.* 
The great Silurian tract which extends from St Abb’s Head to Portpatrick 
consists, as is well known, of flexured grits and slates, often much shattered and 
broken, and almost everywhere traversed by dykes of various felstones, and 
* Since the earlier part of this paper was written, I have had an opportunity of examining the 
geology of the east part of Berwickshire ; especially the great ashy series in the neighbourhood of 
Reston, and along the coast from near Coldingham to Eyemouth. The structure of this region is 
often very obscure. It is possible that some of the felspathic conglomerates of the Ale Water may 
belong to the Lower Silurian series, but the greater part, if not the whole, of the ashy rocks of this 
district must, I think, be regarded as of Lower Old Red Sandstone age. Near the village of Reston 
I have found, in some shales in the ashy strata, remains of plants and broken crustacean frag- 
ments, which my colleague, Mr Salter, is inclined to consider as belonging to the Pterygotus. These 
strata are covered unconformably by the Upper Old Red Sandstone. 
VOL. XXII. PART III. 8c 
