148 HKpwarp Hutt—The Relations of the Carboniferous, Devonian, and Upper 
the Ilfracombe and Lynton beds are those of the Lower Carboniferous series 
repeated by faults,“ so I am equally unable to accept Mr. Etheridge’s view that 
they are represented by the Dingle and Glengariff Beds.? Against the former 
view we have not only the paleontological evidence (which to my mind is 
conclusive) that the Ilfracombe beds cannot be the equivalents of the Marwood, 
Pilton, and Barnstaple beds, but we have also the undoubted superposition of the 
Pickwell Down Sandstone on the Morthoe slates, as seen near Morthoe station, { 
and Exmoor.§ Against the latter we have the extreme difficulty of supposing that 
a highly fossil-bearing group of strata of great thickness in Devonshire could be 
represented by an unfossiliferous group of great thickness and of different mineral 
characters in the closely adjoining district of the south of Ireland. It is not till we 
reach the bottom of the whole series that we really meet with the representative 
beds in Devonshire, but the overlying fossiliferous beds have really no representa- 
tives over the Irish area. It is here, in fact, that the great hiatus occurs, owing to 
which the Old Red Sandstone is everywhere unconformable upon whichever forma. 
tion it happens to rest. Thus it is that the missing chapter between the Silurian 
and the Carboniferous in the paleontological history of Ireland is supplied by the 
rocks of Devonshire with their teeming populations of marine organisms, and the 
Devonian rocks assume their true proportions in the geological series of the British 
Isles, and offer a key to unlock one of the problems of Irish geology. 
VII.—Geoyraphical Considerations. 
The whole subject we have been considering forces upon our view a remarkable 
series of changes of land-surface and sea-bed ;—successive phases of elevation and 
depression of the southern portions of the British Isles, which I shall now attempt 
briefly to point out. 
The Upper Silurian rocks appear to have been deposited in depressions and 
valleys formed out of the Lower Silurian rocks which had been disturbed, elevated, 
sometimes metamorphosed, and greatly denuded at the close of the Lower Silurian 
period.|| Upon the re-submergence of the land at the commencement of the Upper 
Silurian period beds of conglomerate, breccia, grit, and slate were formed during 
the “Upper Llandovery ” period—to be followed by finer sediments, sometimes 
with calcareous bands, and terminating upwards with the great group of rocks we 
have described under the name of ‘‘ Glengaritf” or “ Dingle Beds.” The maximum 
depression of the sea-bed in the south-west of Ireland must have amounted to nearly 
20,000 feet ; that is to say, an amount more than sufficient to bring the summit of 
the Alps to the level of the sea. The amount of the depression was probably much 
* Professor Jukes only assumed the existence of one repeating fault, but it seems to me that to 
account for the Lynton Beds, according to his view, two are necessary. + Supra cit., Table XIT., p. 698. 
{ Supra, p. 146. § Mr. Ussher, Geol. Mag., February, 1879, p. 93. 
|| Physical Geology of Ireland, p. 21, et seq. 
