Ragnarok : the age of fire and gravel

WHAT IS A COMET? 79

must conceive of some way whereby, as soon as the sand is formed, it is removed from the stones while the work of attrition goes on. This process we can conceive of ina comet, if the finer detritus is constantly carried back and arranged in the order of the size of its particles.

To illustrate my meaning : let one place any hard substance, consisting of large fragments, in a mortar, and proceed to reduce it with a pestle to a fine powder. The work proceeds rapidly at first, until a portion of the material is triturated ; you then find that the pulverized part has packed around and protected the larger fragments, and the work is brought to a stand-still. You have to remove the finer material if you would crush the pieces that remain.

The sea does not separate the sand from the gravel ; it places all together at elevations where the waves can not reach them :

“ Waves or shallow soundings have some transporting power ; and, as they always move toward the land, their action is landward. They thus beat back, little by little, any detritus in the waters, preventing that loss to continents or islands which would take place if it were carried out to sea.” *

The pebbles and gravel are soon driven by the waves up the shore, and beyond the reach of further wear ; + and “the rivers carry only silt to the ocean.” {

The brooks and rivers produce much more gravel than the sea-shore :

_ “The detritus brought down by rivers is vastly greater in quantity than the stones, sand, or clay produced by the wear of the coasts.” *

* Dana’s “Text Book,” p. 288. + Ibid., p, 291. t Ibid., p. 302, * Thid., p. 290.