Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/268

 Here and there ridges of Boulder-clay and other kinds of drift aim northwards ; and all the loose stones in these hills of drift seemed to be granite of the country. At Dungloe is a considerable ridge of granitic boulder-clay, parallel to the course of the river and to the fjord. Where the sea has newly washed this clay from the rock, glacial surfaces are perfect *.

The marks aim from the foot of Slieve Snaght, at the north end of Arran Island. Followed in that direction the marks still aim out to sea northwards and to the west of north. Prom rising grounds near the sea, Arrigle and Mookish are seen above the granite lowlands. At the base of the high cone of Arrigle are great blocks of grey granite resting upon the quartz ; and all along the sky-line seen from Glen Veagh and from the Gweebarra Pass great stones are perched. The solid granite is in the S.E. ridge of Glen Veagh, quartz is on the N.W. side of that furrow ; and the granite boulders seem to have crossed from the Slieve Snaght ridge towards the N.W.

All recent glacial marks that I could find in this region indicate a local system of Donegal glaciers which moved as ice now moves about Bergen and in Iceland. In particular the ridge which divides Glen Veagh from Gweebarra Pass was covered by a sheet of ice like Lang Jokull in Iceland, which flowed off it as water flows off the roof of a house into gutters. The ice, according to its marks, once was about 2000 feet thick, and went out to sea ; but it dwindled and shrank till nothing remained but river-glaciers, and then mouutain- glaciers of the smallest size, with one of which I began. (XII. 1.)

6. Irish local systems. — That which is true of Donegal is true of all the groups of mountains which I have visited in Ireland.

A smaller local system left conspicuous marks on the peninsula which ends in Slieve Liag, wbich I have already mentioned (XL). Another was in the Antrim mountains. There moraines are entirely made of fragments of rocks of Antrim. Whole walls are built of boulders of basalt ; and the Boulder-clay is brown. Glacial marks upon the rocks follow the run of water, beside rivers, from the snow-shed downwards. Prom Lame to Ballyeastle the Antrim glens were filled with glaciers like those of Donegal, which were like those of Iceland. Another large local system was in the Mourne Mountains. Another had the Twelve Pins and other hills of Connemara for gathering-ground and starting-point.

Another was in the group of hills on the west side of Lough Neagh ; another was in the Sligo hills ; another was out near Achill Head. A very large system was in the south-western corner of Ireland, with the high grounds about Killarney for gathering-ground and the sea for receptacle. This last has been described by Mr. Close, and by other writers.

Knowing something of all these systems, and of others of less size, it is proved, by marks about which there can be no doubt, that Irish glaciers, down to latitude 51°, were equal in area and dimensions to the largest local ice- systems in Iceland, which touches the Arctic circle. But the Irish ice-system was still larger at an earlier time.


 * Specimens of glaciated granite were shown.