Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

ADDENDA. 619 I will now endeavour to show how far these very curious facts, which we owe to M. Agassiz's observation, can be explained by the theory of floating ice : and the theory, I may add, if applicable to this case, solves that wliich presents far greater difficulties, than any other of the kind in Europe. I should first state that I make two assuwptio7is, and if these be rejected, the theory is not applicable to the case of the erratic blocks of the Alps : — first, that an arm of the sea extended between the Jura and the Alps, during that period in which, as I have before shown, it is'probable, that the proportional area of water in Europe was greater, and certain, that the productions of the land and water had a more tropical character, at the same time that the snow-line descended lower. The age of the Molasse, which occupies this area, between the Jura and Alps, has not been accurately determined ; but it is supposed to be miocene, and is said to contain leaves of the Chamserops, a genus of palms, at present found further from the equator than any other kind. It is not, however, evident that the Molasse was deposited by the sea during the latest period, when it occupied a confined limit between the Alps and Jura ; but even if this should be found to be the case, it would be rash in any one positively to conclude that glaciers could not have descended to the shores of a sea, on which the Chamasrops flourished, after that we know they descend in the southern hemisphere so near the limits of several tropical forms. My second assumption is, that the elevation of this much of Switzer- land, whenever it took place, was slow and gradual : this is supported by the strongest analogies of South America, Scandinavia, and other parts of the world ; whilst on the other hand, the assumption that it had been sudden, would be unsupported by a single ascertained fact in nature. Now, as the numerous masses of ice, which fall from the glaciers at the head of the sounds on the South American coast, are slowly drifted outwards (owing to the fresh water flowing in from the foot of the glaciers), and in the more open channels are left to be acted on by the winds and currents; so must it have been with the icebergs from the glaciers of the Alps, situated in the same latitude, and under similar conditions. These icebergs would in most cases be driven on some part of the surrounding shore ; but from floating deep they would ground a little way from the beach, and then being packed together, and driven to and fro, as the winds changed, and as the tides rose and fell, would they not, like a glacier on the land, though in a lesser degree, "bruise and grind down every thing and polish the solid surface, on which they reposed?" In the rapids of the North American rivers, over which large bodies of ice are driven, carrying with them peb- bles and fragments of rocks, I am informed by Dr. Richardson, that the primitive rocks are scooped and hollowed, and have their surfaces polished and glossy. Dr. Richardson, however, is not prepared to say, whether this