origin, has been raised over the narrow pass in the hills above, from a height of 500 to one exceeding 1,400 feet. For these and many other cases mentioned by other authors, Mr. Darwin offers the ingenious explanation afforded by shore-ice, formed during a general and continual subsidence of the land over large areas. By this combination, the boulders on the shore might be frozen and refrozen at levels necessarily higher and higher, as compared with the land, drifted and redrifted in floating ice, and subject to more or less of rolling and attrition, till in small numbers, in limited tracts, and under peculiar geographical conditions, they might assume the paradoxical situations for which mere sea currents cannot account.[1]
The supposition that erratic blocks have been transported by floating ice, leads to an admission that much of the surface of the northern circumpolar regions, which is now dry land, was under a sea periodically chilled by abundance of ice, if not placed permanently on lower isothermal bands than at present. It is an equally clear inference that the lands from which the ice-rafts started—for example, the Scandinavian and British Highlands—were extensively overspread by glaciers. But they were, by the hypothesis, at a lower level; and it becomes necessary to assign a reason for their greater cold under a condition which in the given examples is actually favourable to augmented temperature. As things are, if our British mountains were lowered a few hundred feet, their summits would grow warmer, and there would be even less chance than at present for the production of glaciers on them. By the same depression the Scandinavian glaciers would be contracted, The cause is probably given correctly by Mr. Hopkins,[2] and is simply the displacement of the northward current of warmed water ('gulf stream'), which, bathing the shores of Britain and Norway, exalts their temperature at present about 15° above that of the corresponding latitude on the east coast of North America. This gulf
