Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/278

 was more than 2000 feet thick when it went to sea. Ice of that thickness would slide along the sea-bottom till it reached water 1800 feet deep (300 fathoms). Between Ceantire and Fairhead the deepest sounding given is 456 feet ; between May and Instrahull the deepest is only 312 feet.' The limit of 600 feet (100 fathoms) passes far outside the British isles. The limit of 1800 feet is out about Rockall. According to charts, the bottom is chiefly made of sand and shells and mud, the light "drift" which tides pack in harbours. But the lead finds rocky hills and deep hollows under water, and trawlers have fished up great boulders south of Plymouth.

Late changes in the level of sea and land are proved by raised sea-margins, by sea-shells far inland, and by submerged peat in the south of Ireland and off Wales. But if the sea were 1200 feet deeper than it is, glacier ice 2000 feet thick might still slide along the sea-bottom from Scandinavia to Kerry.

XXI. Landscapes of the glacial period, which I now picture to myself still, are but magnified images of real landscapes. In Greenland and in Spitzbergen, and notably in south-polar regions, very thick broad sheets of ice slide off land into the sea. These crusts do not end suddenly at the water-level ; they break or they slide along the bottom like rafts on slips till they get out of their depth. The ice-rafts meet and join like glaciers on shore in shallow straits about Greenland. Such rafts enlarged would unite if they met in mid ocean. It is well known that glaciers on shore are forced over hills by sufficient pressure from higher hills or from higher snow-heaps. The same glacier partially floated by water can be driven over sunken bills by less force.

Terrestrial and amphibious glaciers are ice, and have greater mobility for weight decreased by partial flotation. Awash in moving water and aground, a glacier pushed seaward is easier to move and is moved by more forces. Ice that slides off Greenland south-eastwards and north-westwards is turned south-westwards by the Arctic current. It would be affected by that current if it were aground all the way to Iceland and Labrador. Were the Arctic current transferred to the Baltic, and Scandinavian ice enlarged till the whole of that sea was one wet glacier, the laws which govern the circulation of ocean-currents would not be repealed. If the local systems of Greenland, Iceland, and Scandinavia were united in the Atlantic and aground in its shallows, water left fluid in deeps and further south would still move in obedience to existing laws, and would still move ice adrift or awash in it, however large and deep the ice might be. Glaciers did move through hollows which are lake-beds now. I suppose that a far larger glacier than any now extant moved along the beds of shallow seas. In striving to picture the glacial period I invent nothing ; but I strive to shake off ideas of size. The whole world is a very little thing to the solar system ; but so far as we know, the same mechanical laws govern the movements of the whole machine and all its parts. The authors of the ' Reign of Law' and of the ; Theory of Lakes' will agree with this.

XXII. Ice-marks. — If I reduce a country on the scale of a mile