558 A TREATISE ON METAMORPH1SM. 



well known, by far the most abundant of all the constituents thus precipi- 

 tated is calcium carbonate. The corals and other shell animals abstract 

 this material from the water and build it into their hard parts. As the 

 animals die this material is deposited, and thus great limestone formations 

 are built up, and doubtless have been in the past. No adequate reason 

 has been given for the belief that great seas connected with the ocean have 

 become saturated by calcium carbonate so as to build up limestone forma- 

 tions by chemical precipitation, although, as already intimated, chemical 

 precipitation of calcium carbonate has locally occurred. While the chief 

 precipitation of calcium carbonate is through the agency of life, chemical 

 replacement works in conjunction with organic precipitation. No sooner is 

 a deposit of calcium carbonate formed than the other salts of the sea begin 

 to act upon it, under the laws of heterogeneous systems, and thus modify 

 the deposit. Calcium is taken in solution and is replaced by the other 

 bases, especially by magnesium. (See pp. 798-802.) 



MATERIAL TRANSPORTED IN SUSPENSION AND SOLUTION.. 



Commonly the material transported in suspension and that transported 

 in solution are built into deposits which are largely separated from each 

 other; but not infrequently both classes of material are laid down together, 

 and this causes additional variety in the sedimentary rocks. We thus have 

 sediments built up of all proportions of materials mechanically transported 

 and materials chemically transported. It therefore appears that the sedi- 

 mentary rocks may have a wider variety of chemical composition and of 

 mineral composition than any of the igneous rocks. But while this is 

 possible, the rule for the great sedimentary formations is simplicity, and 

 this is due to the tendency toward simplicity in the belt of weathering and 

 to the processes of assorting. The first great process of assorting in the 

 belt of weathering is the subdivision of material into insoluble compounds 

 transported in suspension and soluble compounds transported in solution. 

 As already pointed out, the mechanical sediments are commonly assorted 

 by waves and currents, and the chemical sediments are assorted by life; 

 and thus the dominant formations of the sedimentaiy rocks are simple, both 

 chemically and mineralogically. This is especially true of the two 

 kinds of formations, sandstones and limestones. The shales, arkoses, 

 graywackes, and conglomerates represent the varieties of sedimentary 

 rocks in which there is greater complexity. 



