Page:The American Cyclopædia (1879) Volume II.djvu/83

 ATLANTIC OCEAN 71 fathoms in the western and 2,200 in the east- ern ; and the Black sea a depth of 00 to 900 fathoms. The whole Mediterranean'system is separated from the Atlantic by a barrier of 150 to 200 fathoms at the strait of Gibraltar. The Caribbean sea is deep, reaching to about 2,500 fathoms in some parts, and the passages between the Windward Islands are in some places more than 1,000 fathoms. The passage through the strait of Yucatan has about the same depth, and the gulf of Mexico may reach 2,000 fathoms in its central part. Its com- munications with the Atlantic through the strait of Florida and the Old Bahama channel do not exceed 400 or 500 fathoms. From what we know at present of the Atlantic ocean bottom, it appears to be entirely destitute of any submarine chains of mountains analogous to those we have on land ; there are no steep valleys, no bare rocks, in fact none of that variety of surface which on dry land contrib- utes so much to the beauty of the scenery. For incalculable ages a slow but permanent shower of organic debris has been descending from the surface, which, mingling at the bottom with the skeletons of its inhabitants, has formed a uniform layer of a soft calcareous ooze of un- known thickness, covering the accidents of the bottom as a snowstorm levels the hillocks and ditches of our fields. Being entirely unaffected by changes of temperature and of moisture, the ocean bottom cannot show the effects of weath- er or of erosion, the magnitude of which on the terrestrial relief is as yet greatly under- rated even by many geologists. It is only in the northern parts of the ocean (and probably in the southern also) that in a certain sense the traces of atmospheric action on the surface of the bottom can be found, but only mediately. The banks of Newfoundland are, if not formed, at least increased by the sand and pebbles an- nually brought down, though in small quanti- ties, from the arctic regions by the icebergs, of which this is the great melting ground. The rounded pebbles of basalt found by Wallich be- tween the Faroe islands and Iceland, and the gruvel and pebbles observed by Carpenter in the deep-sea dredgings off the Faroes, have probably also an arctic origin, drift ice having been seen, though rarely, very nearly in the same localities. The foregoing remarks apply of course only to the deep-sea basin. On the terrace fringing the continents the force of tidal and other currents has had more effect in shap- ing the bottom ; rocks and coral reefs lift their heads to or above the surface ; in a word, there is more superficial variety, but even here it is sel- dom comparable to many of the subaerial reliefs. III. CONSTITUTION OF THE OCEAN BED. It has always been the practice in navigation to arm the sounding lead, i. e., to fill a cavity at its base with tallow (the arming). Particles of sand, stones, shells, &c., remain attached to it after a cast, and give, by their proportions, color, or size, indications of the position of a ship, frequently of great value. Hydrogruphers have devised more convenient means of bring- ing up specimens of the bottom. In France the sounding lance is mostly used, a pointed bar of iron projecting under the lead, and provided with notches or barbs in which the sand or mud remains. In the United States coast survey the characteristic specimens of bottom are preserved with care, in the first place as vouchers of the correctness of the data given on the charts, and secondly for purposes of scientific investigation. Lieut. Stellwagen, U. S. N., while on coast survey duty, proposed a simple instrument for bringing up specimens, which, under the name of the Stellwagen cup, has been extensively and satisfactorily used. It consists in a conical iron cup, screwed into a rod projecting from the base of the lead, and having its opening covered by a loose leather valve. When the lead strikes, the cup is driven into the bottom and fills, and the pres- sure of the water afterward keeps the cover down while hauling up. A slightly different sounding cup was invented by Admiral Sands, in which the opening into the cup is at the side and kept closed by a spring, which opens only when the cup is penetrating into the soil. In Brooke's sounding apparatus, before men- tioned, the cavity at the end of the rod was at first filled with quills in which the mud lodged ; later a valve was provided which was pressed over the opening by the sliding off of the cannon ball. The quantity brought up in that way was, however, always very small. The greater part of the extensive col- lection of specimens of soundings in the coast i survey office in Washington have been pro- j cured with the Stellwagen and the Sands cups. In England the Bulldog machine, so called, has been successfully used for some years. It is a modification of Capt. Ross's clams, and con- sists of a pair of scoops closing against each other and thus bringing up a considerable quantity of material. The results obtained by these different methods have been laid down in maps, in France by M. Delesse and in Ame- rica by Mr. Pourtales, and thus a general idea of the geology of the bottom of the ocean has been obtained, or rather of its lithology, as M. Delesse has called it ; for under water it is only the superficial layer which is brought to our knowledge ; of its thickness, superposition, &c., the sounding lead can give us no idea. From these researches it appears that on the coast terrace there is, as might be expected, a great variety in the constitution of the bottom. It reflects as it were the geological formations of the adjacent shore, but with this difference, that the movement of the water produces a sifting action when agitated by the tides, winds, or currents, the heavier and harder particles remaining alone in some localities, while the lighter and finer materials are transported and deposited in others. This accounts in part for the immense preponderance of silicious sand in the deposits of the terrace, since it is the result of the decomposition of most of the primitive