II. AQUEOUS EOCKS 



1. BOOKS FORMED THROUGH CHEMIOAJL AGEITCIES 



This comparatively small, tliougli by no means unimportant, 

 group of rocks comprises those substances which, having once 

 been in a condition of aqueous solution, have been deposited as 

 rock masses either by cooling, evaporation, by a diminution of 

 pressure, or by direct chemical precipitation. It also includes 

 the simpler forms of those produced by chemical changes in 

 pre-existing rocks. Water, when pure or charged with more 

 or less acid or alkaline material, and particularly when acting 

 under great pressure, is an almost universal solvent. Thus, 

 heated alkaline waters, permeating the rocks of the earth's 

 crust at great depths below the surface, are enabled to dis- 

 solve from them various mineral matters with which they come 

 in contact. On coming to the surface or flowing into crevices, 

 the pressure is diminished, or evaporation takes place, and the 

 water, no longer able to carry its load, deposits it wholly or in 

 part as vein material or a surface coating. In other cases alka- 

 line or acid waters bearing mineral matters, may, in course of 

 their percolations, be brought in contact with neutralizing solu- 

 tions, and the dissolved materials be deposited by direct precipi- 

 tation. In these various ways were formed the rocks here de- 

 scribed. It will be observed that the various members of the 

 group are composed mainly of minerals of a single species only. 



This group cannot be separated by any sharp lines from 

 that which is to follow, inasmuch as many rocks are not the 

 product of a single agency, acting alone, but are rather the 

 result of two or more combined processes. This is especially the 

 case with the limestones. It is safe to assume that few of these 

 are due wholly to accumulations of calcareous, organic remains, 

 but are, in part at least, chemical precipitates, as is well illus- 

 trated by the oolitic varieties. 



According to their chemical nature, the group is divided 

 into (1) Oxides, (2) Carbonates, (3) Silicates, (4) Sulphates, 

 (5) Phosphates, and (6) Chlorides. 



99 



