MATERIAL OF THE SEDIMENTARY ROCKS. 557 



decomposition. In eases where both the decomposed and the undecom- 

 posed materials are abundant the mineralogical complexity is even greater 

 than in the ordinary original rocks. 



Even where decomposable minerals are largely altered and the 

 abundant minerals are few, by different combinations of these few minerals 

 a wide variety of sedimentary rocks may be produced. This is due to the 

 opposing processes of mingling of the materials in streams and separation 

 of them by waves and currents. Where material is deposited at the mouth 

 of a great and rapid river all the minerals are intermingled; where the 

 material is contributed to the sea no faster than it can be sorted by 

 the waves and the currents the different minerals are separated. Between 

 thorough assorting and no assorting there are all gradations. In proportion 

 as the material is unassorted the combinations of different minerals and of 

 different- sized particles built into a deposit is varied; in proportion as the 

 process of assorting is advanced a deposit is likely to be built up of a single 

 mineral or of a combination of two or more minerals having approximately 

 the same size and the same specific gravity, or at any rate which are 

 floatable to the same degree. Where the materials are slowly contributed 

 to the sea and are long subjected to the waves, the nearly perfect assorting, 

 both as to mineral material and as to size, is remarkable. As an illustration 

 of nearly perfect assorting may be mentioned the St. Peter sandstone of 

 Wisconsin. This sandstone is almost- wholly composed of quartz grains, 

 analyses showing' it to have 96.74 per cent of silica" Moreover, the varia- 

 tion in diameter of the great majority of the grains is less than by ratios 

 of 2:3 (pp. 861-862). 



MATERIAL TRANSPORTED IN SOLUTION. 



It has been pointed out that the material transported in solution is 

 mainly composed of the more soluble compounds, viz, salts of the alkalies 

 and the alkaline earths. The ocean as a whole is not saturated with the salts 

 derived from the belt of weathering, nor is there any evidence that it has 

 been so at any time in the past, although locally inclosed seas and lagoons 

 do become saturated, and in this case chemical precipitation may occur 

 precisely as in inland lakes, described on pages 551-553. But the domi- 

 nant precipitation of material from solution in the ocean is not through the 

 process of chemical precipitation, but through the agency of life. As is 



« Geology of Wisconsin, vol. 2, 1878, p. 680. 



