Page:The American Cyclopædia (1879) Volume II.djvu/82

 70 ATLANTIC OCEAN for the Atlantic, which is probably not far from the truth. Laplace supposed the mean depth of the ocean to be of the same order as the mean elevation of the land. But his supposed mean height of the land, 3,000 ft. (Humboldt estimated it more correctly at 1,000), was much too small to represent the mean depth of the ocean. Among the first connected series of deep-sea soundings were those made by the United States coast survey in connection with the exploration of the Gulf stream, those of Capt. Lee and Capt. Berryman in the brig Dolphin, of Sir Leopold McClintock in the Bulldog, and others. When the projects for laying submarine telegraph cables across the ocean began to assume importance, a sudden impetus was given to deep-sea sounding ; com- plete sections across the ocean were explored in different directions, and the whole subject appeared much less formidable than before. After such feats as finding and grappling suc- cessfully a broken cable in mid-ocean and in nearly two thousand fathoms, the mere fact of sounding to obtain the depth appeared very simple. In late years a new scientific interest has arisen in the study of the deep-sea bottom by means of the dredge, and numerous sound- ings have been taken in connection with it in Europe and America. In studying a chart of the ocean containing many soundings it will be observed that on leaving the shore, in the greater number of cases, the depth does not increase regularly or according to a uniform slope, but that the bottom forms as it were a terrace around the continents, sloping very gradually down to a certain depth, from which there is a much more rapid descent into deep water. This depth we may assume at about 100 fathoms, and that line is generally marked on the maps ; but it is really somewhat less, probably in the neighborhood of 80 fathoms. We may, for instance, find that we must sail 100 m. from the shore to find 100 fathoms depth ; but in 10 m. more the lead would sink to 1,000. Hence, should the level of the ocean sink 100 fathoms, a large addition of territory would be made to the continents; 100 fathoms more would increase this addition by a mere narrow strip, very steep toward the sea. This terrace probably marks the ancient margin of the continents, and has been gradually formed by the encroachment of the ocean on the land. Hence it is as a rule wider on coasts formed of materials easily disintegrated than on those formed of hard rocks. The terrace is narrow on the coast of Spain and Portugal, and widens largely from the bay of Biscay northward, ex- tending from 50 to 100 m. outside of the Brit- ish islands, which it embraces together with the whole North sea. It is narrow along the coast of Norway, but extends from Spitzbergen half way to Cape North. On the coast of North America it is very wide, though inter- rupted at several points, from Newfoundland to Cape Cod, embracing all the banks. South of Cape Cod it is from 60 to 100 m. broad, narrowest at Cape Hatteras and tapering off toward Florida, but wide again on the W. side of this peninsula. The West Indies gen- erally rise out of deep water. The terrace along the coast of South America varies gen- erally from (iO to 100 m. in breadth, but be- comes much wider S. of the Rio de la Plata, so as to include the Falklands. At the Cape of Good Hope it extends about 100 m. S. It has not yet been developed by observation along the W. coast of Africa. With regard to the depth of the trough of the South Atlantic ocean, we have little information. Some of the supposed deepest soundings on record, from 7,000 to 8,000 fathoms, were made off the coast of South America, but they are entirely dis- credited now. From a few trustworthy ones it is fair to suppose this basin to have what is probably the average depth of all oceans, viz., from 2,000 to 3,000 fathoms. (It may be stated in passing, that for the Pacific ocean the aver- age depth between Japan and California, de- duced from the velocity of earthquake waves, was found a little over 2,000 fathoms, between Chili and the Sandwich Islands 2,500, and be- tween Chili and New Zealand only 1,500 fath- oms.) Of the North Atlantic more is known than of any other ocean. The lines of sound- ings taken from England and France to New- foundland, for the telegraph cables, show that no depth in that part exceeds 2,400 fath- oms. From these and other soundings it ap- pears that the bed of the North Atlantic con- sists of two valleys separated by a broad ridge running from the Azores to Iceland. The depth over the ridge is always less than 2,000 fath- oms, generally about 1,500; it widens and shoals toward the north, forming there a wide plateau embracing both Iceland and the Faroe islands, with a depth of little more than 300 fathoms. The eastern valley varies between 2,000 and 2,500 fathoms, seems to extend to the equator, and shoals and tapers toward the north, turning at the same time toward the northeast, until it is reduced to the narrow channel between the Shetland and Faroe isl- ands, with 600 fathoms. Beyond this point it cannot be followed for want of data. The western valley is not well known in its south- ern and middle part. It is probably very broad in the great bay formed between the West Indies, the United States, and Newfound- land, depths of over 3,000 fathoms being re- ported S. of the Bermudas. Very deep water, 4,580 fathoms, is said to have been found a short distance S. of the Grand Bank of New- foundland, but this has not yet been corrobora- ted by additional soundings. The valley then passes E. of the banks, gradually shoaling, and, after sending an offset into Davis strait, passes into the Arctic ocean through the narrow pas- sage between Iceland and Greenland, having there a probable depth of a little more than 1,000 fathoms. Of the seas communicnting with the Atlantic, the Mediterranean in its two basins reaches a depth of about 1,600