A remarkable confirmation of this theory was given during the two or three years after the great eruption of Krakatoa, near Java. The volcanic débris was shot up from the crater many miles high, and the heavier portion of it fell upon the sea for several hundred miles around, and was found to be mainly composed of very thin flakes of volcanic glass. Much of this was of course ground to impalpable dust by the violence of the discharge, and was carried up to a height of many miles. Here it was caught by the return currents of air continually flowing northward and southward above the equatorial zone; and since, when these currents reach the temperate zone, where the surface rotation of the earth is less rapid, they continually flow eastward, the fine dust was thus carried at a great altitude completely around the earth. Its effects were traced some months after the eruption in the appearance of brilliant sunset glows of an exceptional character, often flushing with crimson the whole western half of the visible sky. These glows continued in diminishing splendour for about three years; they were seen all over the temperate zone; and it was calculated that, before they finally disappeared, some of this fine dust must have travelled three times round the globe. The same principle is thought to explain the exquisite blue colour of the deep seas and oceans and of many lakes and springs. Absolutely pure water, like pure air, is colourless, but all seas and lakes, however clear and translucent, contain abundance of very finely divided matter, organic or inorganic, which, as in the atmosphere, reflects the blue rays in such quantity as to overpower the white or coloured light-reflected from the fewer and more rapidly sinking particles of larger size. The oceanic dust is derived from many sources. Minute organisms are constantly dying near the surface, and their skeletons, or fragments of them, fall slowly to the bottom. The mud brought down by rivers, though it cannot be traced on the ocean floor more than about 150 miles from land, yet no doubt furnishes many particles of organic matter which are carried by surface currents to enormous distances and are ultimately dissolved before they reach the bottom. A more important source of finely divided matter is to be found in volcanic dust which, as in the case of Krakatoa, may remain for years in the atmosphere, but which must ultimately fall upon the surface of the earth and ocean. This can be traced in all the deep-sea oozes. Finally there is meteoric dust, which is continually falling to the surface of the earth, but in such minute quantities and in such a finely-divided state that it can be detected only in the oozes of the deepest oceans, where both inorganic and organic débris is almost absent.