38 



PHYSICAL GEOLOGY 



as indicated above. Cemented gravel is conglomerate (Fig. 

 16), while if the pieces are angular instead of roundish, the 



FIG. 16. Conglomerate. About % natural size. (Photograph by Baker.) 



rock is known as breccia (Fig. 17). Cemented sand is sand- 

 stone. The common sandstone cements are lime carbonate, 

 the iron oxides, and silica. The nature of the cement influences 



the color and strength 

 of the sandstone. 

 Quartzite is a dense and 

 very hard rock, pro- 

 duced when the pores 

 of a sandstone are 

 completely filled with 

 quartz. The sand 

 grains of the sandstone 

 are worn fragments of 

 quartz crystals, and 

 the quartz molecules 



FIG. 17. -Quartzitic Breccia. (Neal.) deposited about them 



arrange themselves in accordance with the internal structure 

 of quartz crystals (p. 21), and seek to develop again the 



