The suggestion has been made by several geologists that the Atlantic is not a true ocean like the Pacific, but rather a basin of secondary formation and comparatively recent origin. Moreover it has been shown that, meteorologically, the Atlantic should be considered as an inland sea — not a true ocean. Mr. Austin H. Clark has recently discussed the same matter from the biological stand point. We premise that an inland sea is, biologically speaking, a more or less enclosed body of water that is connected with an ocean and has received all of its fauna from that ocean. Its fauna is therefore composed of the same types that occur in the ocean with which it is connected, 'except that the less plastic and adaptable types may have been eliminated and the remainder modified in proportion to the difference between the physical and chemical conditions of the inland sea and that of the parent ocean. After a protracted statement of the evidence, Mr. Clark concludes: *Thus we are justified in assuming that, so far as the evidence to be deduced from the study of the recent crinoids permits us to judge, the Atlantic Ocean is biologically, as it is geologically and oceanographically, an inland sea, for it has received its entire fauna from outside, by means of four different routes, only two of which are now open, and has never originated any type within its own basin. Furthermore it is an inland sea largely, if not entirely, formed through the unequal subsidence of a land mass; for the fauna of the Caribbean region, composed of ancient and generalized types, appears to have become established before the intrusion of the species from the Mediterranean region, which are more modern and more specialized, became possible?) Depths.— The most profound depression
yet discovered in the Atlantic is the 4cleep n north of Porto Rico, measured by scientists of the coast survey steamer Blake (4,561 fathoms or 27,366 feet-708 fathoms less than that given by soundings near Guam in the Pacific). Recurring now to the two great undersea valleys or troughs we have mentioned, we note that the mean depth of the east valley is about 14,000 or 15,000 feet, while the maximum depth of the west valley or trough is 16,800 feet. The submarine ridge separating these troughs is submerged to a depth of 1,600 fathoms from the Azores to the latitude of the Hebrides. It then rises gradually, emerg ing and culminating in Iceland. The south Atlantic has a slightly greater average depth than the north Atlantic.
Bibliography.— Albert I, Prince of Monaco, 'Oceanography of the North Atlantic' (Geo graphical Journal, Vol. XII, London 1898) ; Clark, A. H., 'The Atlantic Ocean Biologically an Inland Sea' (Leipzig 1914) • Hull, E., 'Monograph on the Sub-Oceanic Physiography of the North Atlaritic Ocean,' with a chapter on the sub-oceanic physical features off the coast of North America and the West Indian Islands, by Prof. J. W. W. Spencer (London 1912) ; Murray, J., The Floor of the North Atlantic Ocean' (American Geographical Society Bulletin, Vol. XXXVII, New York 1905), and, with Hjort, J., and others, 'The Depths of the Ocean) (London 1912) ; Reclus, R., 'The Ocean,' etc. (New York 1873); Schott, G., 'Geographie des Ad. Ozeans' (Hamburg 1912) ; Thomson, C. W., 'The Depths of the Sea' (London 1874).