434 



GEOLOGY, 



nally loose sand beds become sandstones; what were soft muds become 

 shales or limestone, according to composition; what was gravel becomes 

 conglomerate; what was chipstone becomes hreccia; what were volcanic 

 ashes, cinders, and lapilli become tuffs; and what were masses of 

 volcanic blocks and coarse fragments become agglomerates. 



Fig. 354. Fig. 355. 



Fig. 354. — Quartz crystal enlarged by secondary growth. The shaded outline repre- 

 sents the outline of the sand grain; the solid lines, the outline after secondary 

 growth. Magnified 67 diameters. (Van Hise.) 



Fig. 355. — Sandstone and quartzite texture. The shaded outlines represent the 

 surfaces of the sand grains before growth, the intervening white portions, the 

 added quartz, and the black portions, unfilled spaces. Open spaces characterize 

 sandstone. When the spaces are filled with quartz, the rock becomes quartzite. 

 Magnified 35 diameters. (Van Hise.) 



The cementing process works at times in specially interesting ways. 

 In quartz sandstones, the grains are worn fragments of quartz crystals, 

 formed originally in quartz-bearing rock. The crystalline force in these 

 remnants controls the arrangement of the new molecules of silica de- 

 posited about them. The result is that the new deposits tend to 



