In the first place, then, the testimony of the deep-sea soundings may be summarized in a few words. Thanks chiefly to the expeditions of the British and American gun boats, "Challenger" and "Dolphin" (though Germany also was associated in this scientific exploration) the bed of the whole Atlantic Ocean is now mapped out, with the result that an immense bank or ridge of great elevation is shown to exist in mid-Atlantic. This ridge stretches in a southwesterly direction from about fifty degrees north towards the coast of South America, then in a south-easterly direction towards the coast of Africa, changing its direction again about Ascension Island, and running due south to Tristan d'Acunha. The ridge rises almost sheer about 9,000 feet from the ocean depths around it, while the Azores, St. Paul, Ascension, and Tristan d'Acunha are the peaks of this land which still remain above water. A line of 3,500 fathoms, or say 21,000 feet, is required to sound the deepest parts of the Atlantic, but the higher parts of the ridge are only a hundred to a few hundred fathoms beneath the sea.
The soundings too showed that the ridge is covered with volcanic débris of which traces are to be found right across the ocean to the American coasts. Indeed the fact that the ocean bed, particularly about the Azores, has been the scene of volcanic disturbance on a gigantic scale, and that too within a quite measurable period of geologic time, is conclusively proved by the investigations made during the above-named expeditions.
Mr. Starkie Gardner is of opinion that in the Eocene times the British Islands formed part of a larger island or continent stretching into the Atlantic, and "that a great tract of land formerly existed where the sea now is, and that Cornwall, the Scilly and Channel Islands, Ireland and Brittany are the remains of its highest summits."