How enormous must be the magnitude of those ponderous masses may be learned from the fact that the bulk of ice below the level of the water is about eight times as great as that above: and in point of fact, Captain Sir John Ross saw several of them aground in Baffin’s Bay, where the water was 1500 feet deep. It has been calculated that the beds of earth and stones which they carry along cannot be less than from 50,000 to 100,000 tons in weight. Sir Charles Lyell, writing in 1865 from the results of the latest investigations on this subject, says: “Many had supposed that the magnitude commonly attributed to icebergs by unscientific navigators was exaggerated; but now it appears that the popular estimate of their dimensions has rather fallen within than beyond the truth. Many of them, carefully measured by the officers of the French exploring expedition of the Astrolabe, were between 100 and 225 feet high above water, and from two to five miles in length. Captain d’Urville ascertained one of them, which he saw floating, to be _thirteen miles long_, and a hundred feet high, with walls perfectly vertical.”[32] They have been known to drift from Baffin’s Bay to the Azores, and from the South Pole to the Cape of Good Hope.[33] As they approach the milder climate of the temperate zones, the ice gradually melts away, and thus the moraines of arctic and antarctic glaciers are deposited at the bottom of the deep sea. In this way, submarine mountains and valleys and table-lands are strewn over with scattered blocks of foreign rocks, and gravel, and mud, which have been transported hundreds of miles across the unfathomable abysses of the ocean. * * * * * Though we are chiefly concerned with Glaciers and Icebergs as agents of Denudation, yet we cannot pass away from the subject without referring to the Geological theory of an ancient Glacial Period. This little digression from the main purport of our present argument will not be unacceptable, we hope, to our readers. The theory is in itself interesting and ingenious; and it offers an admirable illustration of the kind of reasoning by which Geologists are guided in their speculations.