THE VOLCANO KRAKATOA (SUNDA STRAIT) IN ERUPTION IN 1883.
“Although the deeper sea-floors probably lack mountains,” says Professor Shaler, “they are not without striking reliefs, which, if they could be seen, would present all the dignity which their size gives to the Himalayas or Andes: the difference is that these elevations are not true mountains, but volcanic peaks, sometimes isolated, again accumulated in long, narrow ridges, but all made up of matter poured out from the craters or through great fissures in the crust. So numerous are these heaped masses of lava and other ejections from these vents that there is hardly any considerable area of the oceans where they do not rise above the surface. There are indeed thousands of these volcanic peaks distributed from pole to pole.... Thus on the floor of the North Atlantic there is evidently a long, irregular chain of these elevations extending from the Icelandic group of islands southward to the Azores. If an explorer could view this part of the sea-bottom, he would probably find that the line of craters was as continuous as that exhibited by the volcanoes of the Andes.
“Besides the volcanic peaks,” Professor Shaler continues, “the sea-bottom in certain parts of the tropics ... is beset with the singular elevations formed by coral reefs.” But of these I shall have more to say toward the end of the book, and I allude to them here only as a feature of the invisible landscape beneath the waves.
Over the vast, gently undulating spaces separating these submerged lines of volcanoes and the ridges of coral, lies a mat of mud of unknown thickness, which naturalists term “ooze.” It is principally composed of volcanic dust and of the microscopic “tests,” or flinty limy skeletons of minute animals, few of which are large enough to be seen by the unaided eye. “Dwelling in myriads in the superficial parts of the sea, these foraminifera, as they are termed, sink at death to the bottom, over which they accumulate a thick coating of minutely divided limestone powder, forming a layer of ooze as unsubstantial as the finest snow.”
In regions like the North Atlantic this ooze consists almost wholly of such animal matter; but in other regions, such as the South Pacific, where volcanoes prevail, it is constantly and largely increased by an enormous quantity of mineral matter hurled broadcast by volcanoes, all of which are on islands or near sea-coasts. A part of this is the merest dust, which slowly settles from the air, perhaps hundreds of miles from where it was ejected. A larger part consists of that spongy lava called pumice, which is so full of holes filled with air and gases that it may float half way around the globe before it sinks, as happened after the explosion of Krakatoa.
Into the oceanic ooze, too, sinks so much of all dead fishes and other mid-sea animals as is not dissolved or devoured before reaching it; and it forms the grave of thousands of men. It is often said that ships and other things would not sink far, but would float, suspended by dense water or some miraculous influence, only a few hundred or a few thousand feet below the surface, for no one knows how long. But this eerie notion has no foundation in fact. “No other fate,” we are assured by those who know, “awaits the drowned sailor or his ship than that which comes to the marine creatures who die on the bottom of the sea. In time their dust all passes into the great storehouse of the earth, even as those who receive burial on land.” Wooden wrecks probably last much longer than those of iron.
I have mentioned that a small part of what the sea tears away from the land, or receives from rivers, winds, and other sources, is dissolved in its waters, which now contain, no doubt, samples of every ingredient of the rocks and soils of the dry land, and very likely some elements not yet detected. This solvent power of the sea explains its saltness, and it must go on growing more and more bitter as long as its waves grind at the shores and the rivers run down. The salinity varies in degree, water at great depths being salter than that near the surface, and excelling in saltness where evaporation is rapid, as under the trade-winds, while fresher in the regions of equatorial calms, where an immense amount of rain falls; broadly, the lightest (freshest) water is found at the equator, and the heaviest in the temperate regions. Inclosed, or nearly inclosed, areas become very salt. Thus the Dead Sea is what chemists call a saturated solution, being nearly one third (28 per cent.) salt, and Great Salt Lake in Utah is not far behind. The Red Sea contains 4 per cent., and some parts of the Mediterranean nearly as much. Taking all the open oceans together, about 3½ in every 100 parts (3½ per cent.) is composed of various salts, more than three quarters of which is common salt (chloride of sodium), and the remainder mainly forms of magnesium. One of the Challenger authors has estimated that the oceans contain enough salt to make a layer 170 feet thick over their whole area, and another writer says that the amount, if heaped up, would be four times larger than the whole bulk of Europe above the level of high-water mark, mountains and all.
In early times, indeed, sea-water, which yields about a quarter of a pound of crystallized salt per gallon, was almost the only source of salt for food. Even yet it is the principal source of supply for the manufacture of commercial salt in France, Portugal, Spain, Italy, Austria, the West Indies, and Central and South America; and it is largely used in Holland, Belgium, and Great Britain. The early process, still extensively practised in some parts of Europe, was to admit the sea-water to large partitioned flats floored with clay, where it evaporated rapidly. The salt-crystals remaining were then collected, purified to a greater or less degree, and sold off-hand. It was by similar means that our great-grandfathers in New England and along the Southern coasts provided themselves with salt, only they used large vats arranged over fires instead of earthen basins exposed to the sun.
But analysis of sea-water discloses small quantities of many other recognizable minerals. Silica must be there to supply the needs of many foraminifers, sponges, and other animals; lime in various forms exists, or else such sea animals as mollusks could not compose their shells, nor polyps erect their enormous reefs; bromine is present, and to the iodine and other mineral dyes in the water we owe the lovely purples, crimsons, and scarlets painting corallines, seaweeds, echinoderms, and some molluscan shells, as that of the Sargasso-snail (Janthina).