From these we see that the floor of the ocean is an extensive plain, or series of plains, lying at an average depth of about two and one-half miles beneath the ocean surface. In some places, gigantic mountain ranges rise up from these submerged plains to the very surface of the ocean, or to within points so near the surface that they form dangerous reefs, and volcanic islands.
The depth of the ocean thus varies quite as irregularly and as precipitously as does the level of dry lands in the mountain ranges of Switzerland or South America or India. So far as is officially known in 1911, the greatest depth in the Atlantic Ocean is found between the West Indies and Bermuda, at a point called the Nares Deep, which is 4662 fathoms, or 27,972 feet. The greatest depth, so far discovered in the Indian Ocean, is between Christmas Island and the coast of Java, which is 3828 fathoms, and is called the Wharton Deep.
The greatest depth, so far discovered in the Pacific Ocean is called the Challenger (or Nero) Deep in the North Pacific, which is 5269 fathoms (31,614 feet). To get a comparative idea of this great depth, we can imagine the highest mountain in the world placed in this depth of water, and would then find that the peak of this great mountain would be 2600 feet below the surface of the sea. Thus could Mount Everest be lost in the depths of the Pacific Ocean.
There are, at present on record, fifty-six of these great holes in the sea bottoms which exceed three miles in depth. There are ten areas which lie at a depth greater than four miles, and four places where the depth exceeds five miles.
The depth seems to bear a certain relation to the salinity of the water, for it is found that the amount of salt held in solution is less as the depth increases. This of course is the effect of temperature and pressure changes, as well as the greater quietness of the subsurface waters.
The composition of the salts found in sea-water, that is the proportional amounts of the various component salts, does not vary materially in the different parts of the ocean, although the degree of saturation does vary, as above explained.
The temperature of the ocean varies, at the surface, from 28° F. at the poles, to over 80° F. in the tropics. The cold water, near the poles, at any given point, varies less than 10° F.; and the warm water of the tropics, likewise has a variation, annually, of less than 10° F., in a band that nearly encircles the earth; this band, it is interesting to observe, is the region of coral reefs.
Between these regions of small annual variation, there are two bands surrounding the earth, where the annual temperature variation is greater, and may at some spots even exceed 40° F.
