Page:Popular Science Monthly Volume 10.djvu/573

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HOW THE EARTH WAS EXPLORED IN 1876.

the land was found to be carried out into the sea some hundreds of miles, and clays were being formed, mixed up with the débris of animals. Within a certain distance of the land the deposits, to a great extent, were formed of this material. Over a great part of the North Atlantic there is being deposited the Globigerina ooze—composed, principally, of small chambered shells, extremely minute; and these shells were found in enormous quantities. This deposit is almost entirely of carbonate of lime, and the only rock it could form would be limestone; therefore, over a large part of the North Atlantic, and over many other parts of the world, this limestone is being laid down." These creatures live at and near the surface, and thence the whole of this sort of material at the bottom is derived. "It might be supposed that this formation ought to be as universal as is the distribution of these animals on the surface. Singularly enough, this is not the case. At the depth of 12,000 feet the shells become rotten and yellow; at 13,000 feet there are no shells, and the bottom is one of homogeneous red mud, which, instead of consisting of carbonate of lime, is ordinary clay. I may here interpolate a fact to show how abundant animal life is at or very near the surface of the ocean. The steamer Great Eastern was lately in dock at Milford Haven for the examination of her bottom, which had not been scraped since 1867. Her bottom was found covered with an enormous multitude of mussels, clustered together in one dense and continuous deposit, extending over 52,000 square feet, and which, upon a calculation made, amounted to not less than three hundred tons' weight, or enough to load with a full cargo two ordinary collier brigs.

"Another curious fact observed in the voyage of the Challenger was, that all over the bottom of the sea there is a large quantity of pumice, showing that there are volcanoes, either below the water or otherwise, that are constantly throwing out material. This pumice, which is the froth of lava, is frequently so light as to float on the water, and wherever they were, in any part of the world, they saw it moved about by the current over the surface of the sea. They found living in the sea, on the surface, or just below, a great quantity of beautiful organisms called Radiolarians. They increase with the depth, and many occur at great depths that are not found on the surface at all. The impression formed was that they lived all through the sea, and down to the greatest depths.

"The whole bottom of the Pacific, or the greater part of it, is red clay. The temperature of the ocean at 13,000 feet is very low. It is usually but a little above the freezing-point at the bottom of the Pacific and the Atlantic, and portions of the Southern Sea. The general temperature gradually falls from the surface until the depth of 13,000 feet, below which there is, throughout the sea, a uniform temperature of 37° or 34°, or a little below the freezing-point. The question arose, Whence does the ocean derive this low and uniform temperature? It