182 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Feb. 26, 



peninsula; sea-sliells being found up to these heights imbedded in what 

 look like marine strata. The mere presence of transported boulders, 

 it is evident, can no longer be considered as a sufficient proof of sub- 

 mergence if the existence of an extensive covering of land-ice be 

 once admitted. For the transport of boulders floating ice is doubtless 

 a very efficient cause, but equally so is land-ice ; each case, therefore, 

 must rest on its own merits. The polishing and scoring of the rocks, 

 however, will, I think, be found to have chiefly resulted from the latter 

 cause, at least in this country ; but there are some curious cases for 

 which probably few will be disposed to admit this explanation : such 

 are the north-east strise at the extremity of the Island of Anglesea 

 and on the Isle of Man, proceeding apparently from a cause exterior 

 to these islands; also those parallel to the coast at Bray Head in 

 Ireland, pointing N.E. However improbable it may seem, it wiU be 

 well to bear in mind that it would be possible for land-ice to have 

 caused these markings, supposing it to have been developed to an ex- 

 tent sufficient to fill the bed of the Irish Channel. Without venturing 

 to say that it did so, I merely point out that, had such been the case, 

 it might have marked these rocks in the way we find them to be. 

 The continental ice of Greenland fills areas more extraordinary. 



§ 9. It is therefore very necessary that we should have some criterion 

 whereby we might be able to distinguish glacier- action from the action 

 of floating ice. Mr. Darwin, in an excellent paper on the glaciers of 

 Caernarvonshire, suggested that boss- or dome-formed rocks would 

 probably serve as such. Another circumstance that, I think, should 

 help us is the case of a deep hollow surrounded by a ridge on the 

 side from whence the glacial agent has come. Such a ridge would 

 evidently defend the hollow from the grounding of floating ice coming 

 from that side, but would be of no avail against the erosion by a gla- 

 cier. Now, such instances are frequent in Scotland. The weU- 

 rounded and scored gneiss which I have cited in the deep hollow of 

 the east Kyle of Bute, opposite Colintrive, is one ; that of the Gare- 

 loch, described by Mr. Maclaren, is another ; the remarkable case of 

 the reservoir-lakes of Knapdale may be mentioned as a third. Again, 

 the steadiness of the direction of the striae would seem to be incon- 

 sistent with the action of floating ice, unless in the case of deep- 

 swimming icebergs in an open sea, moving under the steady influence 

 of an ocean- current ; and how could these have grazed the bottoms of 

 our intricate glens ? In the case of the Gareloch, Maclaren found that 

 the bearing of the striae over a length of seven miles does not vary 

 more than a point to the right or left of the axis of the flake, and 

 they are most clearly marked at the lower levels. Now, in Baffin's 

 Bay, Dr. Sutherland teUs us the icebergs tumble about and butt 

 against each other in great confusion, like houses in an earthquake, 

 and also occasionally assume a rotatory motion from the pressure of 

 ice-floes against them. Further, when an iceberg strikes against a 

 sunken ridge, it will push the broken fragments of the rock over into 

 the first deep hollow, and there leave them ; and, if grounded, it 

 would rock about by the action of the surf and thus cause irregular 

 curved markings unlike the straight ones that we always find. 



