72 ATLANTIC OCEAN rooks and of the sifting out of many of the secondary and tertiary formations. Limestones, being generally soft, are reduced to impalpable powder and form deposits of calcareous mud ; while argillaceous mud results from the decom- position of clay slates, marl, and true clay beds. Large pebbles or shingle are rare at a distance from the shore, though common enough on the beaches. They seem to be covered by finer materials, except where swept by currents, as for instance in the British channel, where sev- eral banks of flints from the decomposed chalk beds are known to exist. But besides the de- posits of which we have spoken, resulting from decomposition or remodelling of preexisting ones, there are real formations on a very large scale now going on. The lime dissolved in the sea water is assimilated by organized beings, animals or plants, secreted in solid form, prin- cipally as a carbonate, and, after having per- formed a short duty in the organic world, con- tributes in the form of a new inorganic body to the increase of the earth crust. Thus we see in the vicinity of coral reefs the bottom composed of calcareous mud or sand formed by the disper- sion of corals, shells, and echinoderms, and in shoaler parts largely by the decomposition of lime-secreting seaweeds. This mud or sand often consolidates into hard limestone rock, but more frequently when exposed to the at- mosphere than when it remains under water. But it is chiefly in the deep-sea bed that lime deposits produced by organized beings assume gigantic proportions, at least in horizontal ex- tent. The entire bed of the ocean as far as ex- plored, outside of the coast terrace, is covered by a uniform layer of soft calcareous mud, called ooze by sailors, and composed chiefly of foraminifera, low organisms forming minute chambered shells, and living partly on the bot- tom and partly near the surface, whence they sink after death. With them are mixed the shells of floating mollusks, such as pteropods, of other mollusks inhabiting the bottom itself, the tubes of worms, the remains of bryozoa, echinoderms, corals, &c. Some silica is con- tributed, but in smaller proportions, by anal- ogous process performed by sponges, polycys- tince, and diatomacece. It is, in a word, chalk in process of formation, and has been found throughout the tropical and temperate regions ; in the arctic seas observations are still wanting. Along the coast of the United States the terrace is principally sand. Mud is found in the deep gulf between Cape Cod and Cape Sable, S. of Nantucket, Martha's Vineyard, and Block isl- and, for a distance of nearly 80 m. (Block island soundings), in the so-called mudholes off the entrance to New York harbor, and in a few other localities. A few rocky patches of small extent are found off the New England coast, near New York, and along the coast of the Carolinas. At Cape Florida the sand is re- placed by the coral formation which envelops the southern extremity of the peninsula, and which may be divided into two, the reef for- mation and the deep-sea coral formation ; the former extends from the shores to a depth of about 90 fathoms, but receives its supplies al- most solely from a region between the surface and 10 or 15 fathoms, where the reef-building corals live. The second or deep-sea coral for- mation extends from 90 fathoms to about 300. Beyond this depth, and sometimes even from 100 fathoms downward, the deep-sea ooze or foraminifera mud is found everywhere. IV. CURRENTS. Columbus, according to Dr. Kohl's "History of the Gulf Stream," was the first navigator who observed ocean currents, having noticed that in sounding in the Sargasso sea the lead appeared to be carried away from the ship, a fact which he rightly interpreted by the ship being drifted away from the lead by a surface current. In some of his later voyages he also observed the rapid flow of water through the passages among the Antilles, and the strong currents in the Caribbean sea and on the coast of Honduras. Sebastian Cabot noticed the Labrador current about the same time. The first notice of the Gulf stream, the most important of the currents of the At- lantic, is found in the journal of Alaminos, the pilot of Ponce de Leon in the expedition which led to the discovery of Florida in 1513. Ala- minos, making use of his discovery, led the way in sailing down stream through the strait of Florida when carrying Cortes's despatches from Vera Cruz to Spain. In the narratives of the navigators of the 16th and 17th cen- turies frequent mention is made of the ocean currents, and in particular of the Gulf stream ; it is therefore not a little singular that their details were so imperfectly known as late as the second half of the 18th century that they were rather an impediment than a help to nav- igation, at least for the intercourse between Europe and the northern parts of America. The New England whalers at that time were the best acquainted with the limits of the Gulf stream, and from one of them Benjamin Franklin obtained the information which he published in his chart of that current, intended to point out the most favorable routes between the North American colonies and the mother country. Franklin and Blagden also pointed out the difference between the temperature of the water in the Gulf stream and outside of it. Pownall and Jonathan Williams extended our knowledge of this current; Capt. Strickland remarked its extension further N. and E. than was before suspected, and first argued the exist- ence of the N. E. branch of the Gulf stream, about which there has been so much contro- versy of late. Humboldt and Scoresby also paid much attention to ocean currents, and particularly to the Gulf stream. Finally, Major Rennel undertook the discussion of all the ob- servations of currents, and published the results of his generalizations under the title of " In- vestigations of the Currents of the Atlantic Ocean," a work which remains to this day the principal source of information on the subject.
