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. 



