Page:Popular Science Monthly Volume 89.djvu/211

 Popular Science Moiilhl;/

��197

��Rocks Composed of Diatom Earth Which Float

A NUMBER of rocks arc so light that they float on water. Mtjst of them consist of diatom earth, which is a soft earlh\- mate- rial like chalk, but dilfers from it in being composed chiefly of silica-containing plants mixed with the re- mains of submerged organic growths, or diatoms. Dia- toms flourisii in the surface water of jjarts of the ocean, cspecialh- in the South At- lantic, where they are so abundant as to becloud it and where they ser\'e as food for whales. Their remains sink to the bottom and form great accumulations of dia- tom ooze.

Diatom earth is found in many parts of the world, and is extensively used for polishing. It has been used also as an absorbent in the manufacture of ex- plosives, and as a i)acking about steam boilers. The "silv'er white" of commerce is diatom earth. In the United States it occurs at many localities, of which two

����On the left is a piece of rhyolitic pumice and on the right a piece of hydrocarbon

may be mentioned. Near Richmond, \'irginia, it forms a bed thirty feet thick and one hundred miles in extent; and near Monterey, California, there is a bed of it fifty feet in thickness, but of un- kr.own extent.

When samples of it are subjected to a water bath for hours they seem not to absorb the water. Toaltempt to "water- log" a piece of pumice is foolhardy.

��The Colorado River after one of its overflows when the water has receded and the western sunlight has baked the bed

��A Mud Mosaic in the Wake of the Treacherous Colorado

WHEN the great Colorado River goes on a rampage and o\erflows its banks it deposits vast quantities of mud and sediment. In this way it has built up the enormous rich Colorado delta in Arizona and Southern California, cutting out, through the countless ages, the huge gorge of the Grand Canyon, in many ])laces a mile deep through the rock. The photograph shows what happens to the Colorado River clay, u|)<)ii the recession of the waters. Dr\ing tnider the intensely hot sun, which normalh- reaches one hundred and fifteen to one hundred and twenty degrees in the shade, and cracking into innumerable irregular blocks, it forms a \ast natural mosaic. In some places where the water has stood o\er a flat, this mosaic extends as far as the e>e can distinguish.

The Colorado delta is intensely arid in character, only a few clumps of salt bushes being able to subsist. Where it has been irrigated the yields are enor- mous. The fertility of the soil is almost inexhaustible. The complete harnessing of the Colorado and the utilization of its tremendous flood-How constitute one of the realh- big reclamation engineering problems of the day.

�� �