to a physical force acting independently of any controlling power. Adopting the latter view of ciliary motion, a clever writer has compared the moving diatom to a little steamer with the fires lighted and the paddles going, but without a crew, a pilot, or a captain.
The distinguishing peculiarity of the Diatomaceæ is, that they possess a solid framework of flint, their vegetable matter being merely a delicate investing membrane. The trees of the forest, having passed through their successive stages of development, undergo the process of decay, their constituents being dissipated as invisible gases; but the tiny diatoms are indestructible, and their constantly accumulating skeletons are gradually being deposited in beds beneath the waters which cover three-fifths of the surface of this planet.
"At first," says a celebrated naturalist, "the effect produced by things so small—thousands of which might be contained in a drop, and millions packed together in a cubic inch—may appear of trifling moment, when speaking of so grand an operation as the deposition of submarine strata. But each moment has its value in the measurement of time, to whatever extent of ages the succession may be prolonged; so each of these atoms has a definite relation to space, and their constant production and deposition will at length result in mountains. The examination of the most ancient of the stratified rocks, and of all others in the as-
