556 A TREATISE ON METAMORPHISM. 



MATERIAL TRANSPORTED IN SUSPENSION. 



The dominant constituents of the sedimentary rocks transported in 

 suspension are the minerals produced in and adapted to the belt of 

 weathering' and the slowly decomposable minerals. The first class com- 

 prises the kaolin group, the serpentine-talc group, the chlorite group, the 

 zeolite group, quartz, iron-oxide minerals, and aluminum-oxide minerals. 

 Of the second class the minerals of dominant importance are quartz and 

 the acid feldspars. Quartz is included in both classes, and is the -mineral of 

 greatest abundance in the sedimentary rocks; second to it is feldspar, With 

 the abundant minerals there are present, of course, very subordinate amounts 

 of the very difficultly decomposable minerals, such as garnet, staurolite, 

 tourmaline, zircon, etc. 



The readily alterable minerals are usually rare or subordinate in 

 quantity in the sediments. The group of minerals most readily alterable, 

 the feldspathoids, including nephelite, leucite, and sodalite, are rarely, 

 if ever, found in the sedimentary rocks. Minerals of the olivine group are 

 also rare in the sedimentary rocks. The ferromagnesian group of minerals, 

 including the pyroxenes, amphiboles, and biotites, are more difficultly decom- 

 posable and are frequently found; still, they alter with such readiness that 

 they are generally subordinate in quantity, although locally abundant. 

 While where decomposition has been important the above are the dominant 

 minerals in the mechanical sedimentary rocks, in many sedimentary rocks 

 the materials are but slightly decomposed, disintegration being the chief 

 process of destruction of the original rocks. In mechanical sediments 

 derived from disintegrated material, as already pointed out, all or nearly all 

 the original minerals may be present. Where disintegrated materials are 

 dominant the rocks produced by their consolidation are conglomerates, 

 arkoses, and grits. As would be expected, there is every gradation between 

 rocks in which decomposed material is important and rocks in which disin- 

 tegrated material is dominant. Therefore, sedimentary rocks may be built 

 up of materials which, in chemical composition, stand anywhere between 

 decomposition products and disintegrated products, and which vary from 

 the mineralogical simplicity of the former to the mineralogical complexity 

 of the latter. Since the chemical analyses of such materials vary greatly, 

 as shown by the analyses given on pages 507-515, it is usually possible, by 

 the analysis of a sedimentary rock, to obtain a rough idea as to the stage of 



