The Columbian banks, under which name are comprehended all the banks which begin opposite the eastern shores of the "peninsula of Florida and extend in a south-eastern direction to the Mona Passage between Haiti and Puerto Rico, are divided from the North American banks by the Florida Gulf-stream. They consist of two larger banks, called the Little and Great Bahama Bank, which occupy the north western portion of the group, and of five smaller banks, which occur at great distances from one another in a south-eastern direction. These banks have from 15 to 20 fathoms water on their edges, but they are beset with rocks and numerous shoals, of which a few are dry at low water : they are therefore shunned by vessels. The surface of the pants consists of coral, covered with an accumulation of shells and calcareous sand. On their eastern edges, along the Atlantic, are the Bahama Islands.
Of the smaller banks which occur in the Atlantic we shall only mention the Bank of Arguiu, which lies near the western coast of North Africa, beginning at Cape Blanco N. lat.), and extending to the neighbourhood of Port.endik (about N. lat.). It is dangerous for navigators, as there are many parts iu which the water is not deep enough for largo vessels, many of which have been lost on it. The Abrolhos, near the coast of Brazil, between and S. lat., is not extenaive, but it is extremely dmugeruus on account of the soundings being very irregular, varyiug between 36 and 4 fathoms. In two successive caste of the hand-lesul the soundings frequently vary from 30 to 10, and sometimes even to 4 fathoms.
There are several banks in the Indian Ocean : the most extensive are the Saya de Malta and the Bank of Nazareth. The Saya de Mania extends in length from north to south between 18' and 30', and is cut. by 30' E. long. Tho surface is uneven, and iu many places there are not more than between 6 and 10 fathoms water : it consists of corals and shells. The most southern extreinity of the Nazareth bank consists of the islands called Cargados, in 16' 47' S. lat. and near E. long. From this point the bank extends in a north north-east direction to 14' S. Lat. The surface is tolerably level, mid the least depth of water does not fall short of 14 fathom's.
An extensive lank, called the Needle or Agulhas Bank, surrounds the southern extremity of Africa. It extends towards the west beyond the Cape of Good I lope, and towards the east beyond Cape Padroue, cast of Aigoa Bay. As it is not divided, like the other banks, from the continent by a tract of deeper water, it is not to be con sidered as a table-land, but as a submarine prolongation of the continent. Between Cape Agulhas E. long.) and Cape Vaccas (22' E. long.), it extends to 37' S. Lat., or to a distance exceeding 150 miles from the continent. The depth of water on this bank varies between 30 and 00 fathoms. The surface is composed of coarse sand, corals, shells, and small stones. Along its western edge it is skirted
by a mud-bank, which is covered by water from 50 to 120 fathoms deep. The structure of the laud is thus continued into tho sea, beyond the extremities of continents, in other parte of the world also; as in the islands of Tierra del Fuego at the termination of South America. In other cases, as on the coast of Norway, there is a rapid and com plete cessation of the high ground in the course of a very small distance from the shore.
There are numerous banks in the Pacific, especially south of N. lat. All the larger hanks are surrounded by coral reefs, on which islands of small extent often occur. [REEFS.] Near banks, the sea usually deepens to 200 and 300 fathoms, and even more.
The subject of the depth of the sea in general next claims our attention. We are best acquainted with the depth of several close seas, especially those which are much navigated. The depth of the Baltic is inconsiderable; it usually varies between 30 and 40 fathoms, and only in two or three places sinks below 100 fathoms. The North Sea is somewhat deeper in its northern part. Between the Shetland Islands and the coast of Norway the depth varies between SU and 140 fathoms, but it gets gradually shallower towards the south. In the Straits of Dover the deepest place is only 26 fathoms. The depth of the English Channel increases as we proceed towards the west, but very slowly. East of the Eddystone it does not exceed 50 fathoms. The Irish Channel is in general deeper. Though there are some places between Wales and Ireland in which it does not exceed 40 fathoms, its general depth may be said to vary between 60 and 80 fathoms ; and in the strait between the county of Antrim and that of Wigton in Scotland it attains •100 fathoms. The Mediterranean is much deeper than the Baltic and North Sea, more especially along the southern coast of Spain and about the island of Sardinia, where the depth varies between 500 and 1000 fathoms; but the deepest parts appear to be in the spaces devoid of islands to the west of Sardinia, and to the east of Malta, where the depth is about 1760 fathoms, or two miles. A shallow tract extends from Trapani in Sicily to Cape Bon in Tunis : it is called by the Italian sailors seAcrelti, and is of inconsiderable but very variable depth. That portion of the Mediterranean which lies east of this shallow tract is not so deep as the western part, The Red Sea may be compared iu depth with the eastern part of the Mediterranean. [Rem SEA, iu GEM Div.] The sea which surrounds the islands of the western gulf of the Indian Archipelago seems nowhere to sink much below 50 fathoms, and these islands, in fact, belong to the continent of Asia ; but ou the coast of Northern Borneo, in the Straits of Lonibrok, and near the middle of the Strait of Macassa., commences, southward and eastward, a very deep sea hitherto unfathomed.