From the base of the mountains to the sea is a gentle slope or plain, and this plain extends frequently some distance beyond the shore, forming a continental platform or shelf. The outer edge of this platform is usually at a depth of about 100 fathoms .(600 feet) when the bottom slopes rather sharply— forming the continental slope — to the depths of the ocean. Off the coast of New Jersey the 100-fathom line is 110 miles from shore, while on the coast of California it is only about 10 miles from shore. Great Britain stands on such a shelf and is really part of the continent of Europe. In the same way the East India islands to Celebes are part of Asia. New Zealand may be considered part of Australia.
Permanency of Continents. It is one of the commonplaces of geology that the lands are continually changing; by weathering, by rivers and by the waves of the ocean the rocks are worn away and spread out as sediments on the sea floor. In some parts of the world the land is apparently rising, in others sinking. The northern coast of Norway is rising five feet in 100 years, the coast of New Jersey and parts of the New England coast are sinking. At Boston the rate is about one foot in 100 years, and on the New Jersey coast two feet. Yet geologists, while admitting the instability of the lands, differ as to the permanency of continents. Lyell believed that neither continents nor oceans —so far as any particular part of the earth is concerned — are permanent. His views have been opposed by later geologists, who have thought that while continents may change 'form, certain parts of the earth have always been covered by oceans, while others have not; the Atlantic and Pacific, for instance, were always oceans, while the continents, even if at times partly covered with water, have always been uplifts in the sea floor, or if not always, at least since Archwan times.
It is recognized, of course, that these uplifts have been under shallow, or epicontinental, seas many times. This is shown by the following
facts. In the first place, chalk beds correspond ing to the giobigerine ooze that covers much of the ocean floor are found in the interior of continents; in the second place, the study of fossils has shown that at times in past ages continents now separated by oceans had essen tially the same fauna and flora. The evidence of deep-sea deposits shows that in Cretaceous time a fairly deep sea covered much of Mexico and parts of Texas, New Mexico, Arkansas and Kansas, while another, or perhaps the same wide, deep sea covered southern Europe and northern Africa and extended across Asia to the Pacific. In Cretaceous time, therefore, the continental land masses did not have their present form and arrangement, and theories as to the origin of continents based upon present land forms are at best of doubtful value. The evidence of fossils indicates that in Permian time the Antarctic land mass was of far greater extent than now, and joined South Africa, Patagonia and Australia. Again in Cretaceous time the evidence indicates that Brazil, south central Africa, Madagascar and India were con nected by land masses, so that the Atlantic and Indian oceans had not at all their present shape. There is some reason for believing that no longer ago than the beginning of the Glacial Period the Arctic lands were of far greater ex tent than now, Asia, North America and Europe being connected. Thus, while it is fair to as sume that the position of the continents was determined ages ago, there is nothing per manent about their forms or the present dis tribution of land and water on the earth. See GEOGRAPHY ; GEOLOGY.
Bibliography.-- Mill,