At the bottom of the Atlantic there exists a remarkable plateau, extending from Cape Race in Newfoundland, to Cape Clear in Ireland, a distance of over two thousand miles, with a breadth of four hundred and seventy miles: its mean depth along the whole route is estimated at two miles to two miles and a half. It is upon this telegraphic plateau, as it has been called, that the attempt was made to lay down the cable in 1858, and it is on it that the enterprise has been so successfully completed, during the year 1866. Tubular annelids, capable of boring into all organic substances, are native to this plateau, and have materially assisted in destroying the electric cable. The surface of the plateau had been previously explored by means of Brooke's apparatus, and the bottom was found to be composed chiefly of microscopic calcareous shells (_Foraminifera_), and a few siliceous shells (_Diatomaceæ_). These delicate and fragile shells, which seemed to strew the bottom of the sea, in beds of great thickness, were brought up by the sounding-rod in a state of perfect preservation, which proves that the water is remarkably quiet in these depths,--an inference which is fully borne out by the condition in which the cable of 1858 was found, when picked up in 1866. [Illustration: Fig. 3. Section of the Atlantic, showing its depth and the position of the Atlantic Telegraph.] The first exploration of this plateau was undertaken by the American brig _Dolphin_, which took a hundred soundings one hundred miles from the coast of Scotland, afterwards taking the direction of the Azores, to the north of which bottom was found, consisting of chalk and yellow sand, at nine thousand six hundred feet. To the south of Newfoundland, the depth was found to be sixteen thousand five hundred feet. In 1856, Lieutenant Berryman, of the American steamer _Arctic_, completed a line of soundings from St. John, Newfoundland, to Valentia, off the Irish coast, and in 1857, Lieutenant Dayman, of the English steamship _Cyclops_, repeated the same operation: this last line of soundings, the result of which is represented in the accompanying section, differed slightly from that followed by Lieutenant Berryman.