GKITS. 879 



elements in the secondary rocks as compared with the original rocks is 

 likely to be changed. For instance, alumina may be relatively increased 

 in amount, and sodium greatly depleted, as compared with the potassium, 

 etc., as explained on page 507 et seq. This process of alteration is likely 

 to have proceeded far, in proportion as the material is fine grained. By 

 increase in amount of decomposition and decrease in size of grains, the 

 group of sands under consideration passes by gradation into the muds or 

 pelites. By decrease of the ferromagnesian minerals they pass into the 

 arkose sands. By decrease of the ferromagnesian and feldspathic minerals 

 they pass into the quartz sands. Therefore there are all gradations between 

 the ferromagnesian sands and the pelites, the quartz-feldspar sands, and the 

 quartz sands. 



GEITS. 



Iii the belt of cementation, by consolidation, cementation, and meta- 

 somatism the ferromagnesian sands pass into grits. The grits occupy the 

 same place in the family that sandstone and arkose bear in the quartz- 

 sand, and quartz-feldspar-sand families, respectively. In the induration of 

 the grits consolidation by pressure and welding is comparatively unimpor- 

 tant. The principles of cementation applicable are identical with those of 

 the sandstones and arkoses. A much greater number of cementing* minerals 

 are important in the grits than in either the sandstones or the arkoses. 

 Where a ferromagnesian sand formation extends to the surface in the belt 

 of weathering* there may be contributed to the underground waters all the 

 important elements which make up the great masses of the rocks. Also 

 in the belt of cementation like diversified material may be added to the 

 solutions by alteration of the minerals. Therefore, from the waters per- 

 colating through the ferromagnesian sands almost any of the common 

 rock-making minerals which develop in the belt of cementation may be 

 deposited. While this is true, the fact remains that the most abundant 

 cement is quartz. Next in importance to this, in all probability, is calcite, 

 and occupying a third place are the iron oxides. However, feldspar, horn- 

 blende, and other silicate minerals may be enlarged by each mineral 

 selecting from the solutions appropriate materials for this purpose, precisely 

 as with quartz. Also there may be deposited in the interstices such 

 minerals as the zeolites, epidote, and chlorite, precisely as in the vacuoles 

 of lavas, which have an equal variety of materials. 



