Taking first the prime distinction between land surface and ocean floor, we note that the two differ from one another markedly, alike in their characteristics and in the conditions to which they are exposed. The land is subjected to constantly varying conditions: to the alternation of day and night, and to the changes of the seasons, with corresponding variations in temperature; to the fluctuations of the weather; to running water, and so forth. In the great ocean depths at least, on the other hand, the conditions are remarkably uniform. Neither diurnal nor seasonal changes have here any effect; the temperature seems to fluctuate but little; the water is almost still. This uniformity of physical conditions is reflected in the uniformity of the surface over wide areas. While the land surface shows marked irregularities, the ocean floor has a monotonous character, with more gentle outlines. In its most general form the characters of the sea bottom may be briefly stated. Round the great land masses there is an area of relatively shallow water, which is sometimes only a few miles wide, and at other times extends outwards for hundreds of miles. This region is the _Continental Shelf_, and its seaward boundary for convenience' sake is taken at a depth of 100 fathoms, or 600 feet. Within this zone the influence of the land is still felt, and some of the characters of land surfaces appear. Thus we sometimes find that river valleys are prolonged outwards over the Continental Shelf, giving a markedly irregular appearance to the ocean floor. The British Islands lie upon a Continental Shelf of this kind, and this is one of our reasons for knowing that they are really only a part of the continent of Europe, separated from it by a slight depression. The Continental Shelf slopes away from the land gently, and is widest where it fringes low continents, and narrowest where mountains approach the coast. Over it is spread the waste of the land, the coarser lying near the shore-line, the finer extending outwards to the steep seaward slope. This rapid slope leads down to the more or less uniform ocean plateau, whose surface is broken by the great ocean abysses, the greatest of which has a depth of about six miles. Relative but not absolute uniformity thus characterises all that part of the ocean floor which lies below about 100 fathoms. Again, though the ocean floor is doubtless being slowly raised by the deposition upon it of the oceanic oozes, yet it is also true that as compared with the land surface it displays great constancy. While the land surface is constantly changing owing to the varying forces which act upon it, the floor of the ocean can vary but little from age to age, unless it is acted upon by the internal forces of the earth.