864 A TREATISE ON METAMORPHISM. 



Sandstone is indurated quartz-sand rock. The induration is sufficient 

 to make the grains of sand rather strongly coherent, but not so great that 

 the grains break across when the rock is fractured; on the contrary, the 

 fractures follow the cement around the grains. The change of a sand 

 formation to a sandstone is due to consolidation and cementation. These 

 processes are characteristic of the belt of cementation. The process of con- 

 solidation by pressure is of relatively little importance in sandstones. Slich- 

 ter has shown that sands as naturally deposited by water are likely to have 

 a tolerably compact arrangement, and that mechanical disturbance of such 

 sands is likely to result in a less compact arrangement. Sand grains, while 

 having many more points of contact than psephites, have few as compared 

 with the finer grained sediments. Doubtless the superincumbent pressure 

 does bring these grains into juxtaposition, and may possibly weld them to 

 some extent at the points of contact, but sands in which this process alone 

 has occurred are sure to be very weak. 



The main process of induration is that of cementation. (See PI. IX, 

 A?) Silica is dissolved in the belt of weathering, either by the decomposi- 

 tion of silicates or by the solution of silica, either amorphous or crystal- 

 lized. The solutions join the sea of ground waters, and there the silica 

 is deposited upon the sand grains, usually in optical orientation with 

 them. The result of this process is to partially fill the spaces between the 

 sand grains. These filling materials after a time interfere and thus cement 

 the sand grains together. In so far as they do not interfere, the added 

 material is likely to develop crystal faces, and thus there are produced 

 crystal- faceted sand grains, which together make up the sandstone. Sand- 

 stones the world over are mainly thus indurated, and ordinarily show 

 innumerable crystal faces when a specimen is held so that the light of the 

 sun may be reflected. While quartz is the dominant cement, iron oxide and 

 calcite are often important cements, and many other minerals are unimpor- 

 tant supplementary cements Rocks indurated by cementation are usually 

 called sandstones from the time the cementation is sufficient to make the 

 grains weakly cohere to the stage in which the cementation is far enough 

 advanced to cause the rock to break across the sand grains rather than 

 around them. At this later stage they become quartzite. 



a Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Eept. 

 U. S. Geol. Survey, pt. 2, 1899, p. 305. 



