The fine elastics group consists of clays, shales, mudstones, silts, siltstones, 

 and graywackes. Clays and shales and silts and siltstones are differentiated on 

 the basis of the degree of induration or lithification. For practical reasons, all 

 elastics whose grains are invisible under 12 X magnification are classed as clays 

 or shales. Siltstones, silts, and graywackes are the fine elastics whose individual 

 grains are distinctly visible under 12 X magnification. If the grains are well- 

 rounded and within the limiting grain size, the rock is a siltstone. If the grains 

 are sharp to angular, particularly with feldspars, and are imbedded in a very 

 fine matrix, the rock should be called a graywacke. Some graywackes contain 

 grains much coarser than 0.1 millimeter. 



The term grayivacke has been variously defined, and students of sedimen- 

 tology are by no means in agreement on the limiting composition and physical 

 characters of the group. Inasmuch as the sample examiner is generally limited 

 by low magnification of the microscope and broken rock surfaces, instead of 

 sectioned surfaces, exact identification of constituents is difficult, or in some in- 

 stances, impossible. Therefore, it is advisable to define the rock according to the 

 observable properties. In general, graywacke is a fine to medium-grained clastic 

 rock composed chiefly of angular quartz, with or without feldspar, and rock 

 fragments — all imbedded in a fine matrix of silt, clay, chlorite, and other basic 

 minerals. The larger fragments may have the dimensions of grains in rocks of 

 the coarse clastic group. 



The principal characteristics of fine clastic rocks are given below: 



1. Color: In the subsurface, colors of shales and siltstones are often very 

 significant, either in the correlation of stratigraphic units or in the determina- 

 tion of environments of sediment. It has been amply demonstrated in rocks of 

 different ages in widely separated regions that the colors of shales indicate 

 relative positions in a sedimentary basin. The normal lateral sequence from 

 the shore toward the basin deep is bright red to red and green, to green and 

 gray, to light and dark gray, to dark gray and black. This does not imply that 

 all black shales originated in basin deeps or that all red shales are near-shore 

 deposits. It is, however, a normal arrangement. 



Krynine's analysis of rock colors shows that gray in varying amounts is 

 generally present, even in the bright reds and greens. This is a useful principle 

 to keep in mind when matching the colors of shales with colored pencils. The 

 colored pencils alone are too vivid; but when a hard-lead pencil is applied over 

 the colored area, a close match of the natural rock color is attained. 



2. Composition: The exact chemical or mineralogical composition of a fine 

 clastic rock cannot be determined by the means available to the sample man. 

 However, by simple tests it is possible to distinguish such constituents as the 

 following: montmorillonite, illite, kaolinite (or unclassified clays), calcite and 

 dolomite, oxides of iron, sulphides and sulphates, phosphates, and silica. Certain 

 tests for these constituents are presented later. 



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