892 A TREATISE ON METAMOKPHISM. 



The secondary minerals are important constituents 01 the muds. Of 

 these the most abundant are those which especially form in the belt of 

 weathering, as given on pages 518-521. With these there are, of course, 

 almost all the minerals which form in the belt of cementation, as given on 

 pages 621-627, and finally with both of these classes are residual unaltered 

 minerals, especially refractory ones, such as quartz, orthoclase and micro- 

 cline, albite, etc., and the heavy refractory minerals. Not infrequently 

 the quartz and feldspar constitute the major portion of the material. From 

 the foregoing it is clear that in proportion as alteration is advanced the 

 minerals forming the muds tend toward the very end products of alteration — 

 quartz, kaolin, talc, ferric oxide, and gibbsite. (See p. 520.) Some of the 

 clayey muds are composed almost wholly of quartz and kaolin, with some 

 talc and ferric oxide. It therefore appears that the muds have a wide 

 variety of composition, both chemical and mineral. 



SHALE FAMILY. 



By consolidation, metasomatism, and cementation in the belt of 

 cementation the muds pass into shales. Shale stands, in the pelite order, in 

 the same place in which sandstone, arkose, and grit stand in the psammite 

 order. The processes of alteration are the same in all, but their relative 

 importance is very different in the pelites and psammites. In the induration 

 of the psammites consolidation is of subordinate importance, and cementa- 

 tion by material derived from an outside source is of the first importance. 

 In induration of the muds consolidation due to pressure is of the first impor- 

 tance. The particles are very small. According to Whitney, the finer 

 soils, which in reference to subdivision may be taken as approximately the 

 same as mud, contain from 10,000,000,000 to 20,000,000,000 particles per 

 gram." In a fine sand, such as the St. Peter sandstone of Wisconsin, the 

 number of particles per gram, as determined by Mr. S. H. Ball, are about 

 115,000, and in a somewhat fine grained and better sorted beach sand about 

 167,000 particles per gram. Therefore in a mud, after the steady pressure 

 has squeezed out the water so as to bring the particles in contact with one 

 another, the number of points of contact are from 87,000 to 174,000 times 

 as great as in such a sandstone as the St. Peter, and from 60,000 to 120,000 



"Whitney, Milton, Some physical properties of soils in their relation to moisture and crop 

 distribution: U. S. Dept. Agric, Weather Bureau Bull. No. 4, 1892, pp. 73-74. 



