[16]: In the case of volcanic islands, the upheavals were undoubtedly accompanied by deposits of mud, sand (ocean detritus), marine vegetation, and more or less animal matter, and these organic substances were washed down by the rains into the broken valleys and plains below, when land vegetation almost immediately made its appearance; not because seeds may have drifted thither by any of the different agencies that have been mentioned, but because organic matter can no more help bringing forth life in some form, when conditions favor, than salt water, when exposed to evaporation, can help crystallizing into its symmetrically-arranged salts. And the same would be true of all the coral islands, bringing up the organic matter of the sea to the influence of the light, the rains, and the dews. The islands thus formed in the Pacific Ocean begin to exhibit vegetable life almost as soon as they make their appearance above the reefs, and a line of sea-beach is formed about them. [17]: These, while presenting the most varied and diverse forms of infusorial life, are nevertheless the most constant and abundant type. They abound more or less in all organic infusions. Ehrenberg, however, holds that they are no more animal than vegetal forms. They vary in length from 1/15000 to 1/2000 of an inch, and are consequently too minute to be satisfactorily classified in respect to all their diversified characteristics. [18]: The extent of the southern ice-cap may at least be approximately reached from explorations already made. Capt. Weddell, in 1823, extended his explorations southward to within about 15° of the south pole, where he found an open sea. Capt. Ross, in 1842, approached to within about 13° of the same pole, without serious obstruction. It is true that, in the following year, he encountered ice barriers near the line of the antarctic circle, but they were floating barriers coming down from Weddell's open sea. Capt. Wilkes, in 1840, explored a considerable portion of the Antarctic Continent, lying almost entirely within the antarctic circle. Other explorations have been made, showing that the southern ice-cap does not probably extend, continuously at least, much farther north than 78° or 80°, or to within some ten or twelve degrees of the south pole, independently of the packs of drifting ice in the otherwise open seas.