ALTERATIONS OF ROCKS 285 



the rock, shows the difficulty of defining faults sharply. Many 

 movements of displacement, which can hardly be separated from 

 faults logically, are not usually called faults. 



M>ip -d'ork. The sections of the Structure Section Sheets of the folios of the 

 {' . S. (Irol Surv. furnish abundant illustrations of a variety of structural features, 

 >urh as folding and fault inn. and the relations of sedimentary, metamorphic, and 

 igneous rorks. The sections of various Bulletins, Professional Papers, etc., of the 

 satin- Survey afford other illustrations. See also Exercise XVII in Interpretation of 

 Maps. 



INTERNAL CHANGES IN IGNEOUS AND SEDIMENTARY ROCKS; 

 METAMORPHISM 



We have seen already that igneous rocks undergo physical and 

 chemical changes, whereby they are disintegrated, giving rise to 

 what has been called rock waste: Similarly, sedimentary rocks may 

 be decomposed and converted into waste. The waste from one 

 generation of rock is the raw material for rock of a new generation. 

 It is "rock waste" in somewhat the same sense that lumber is forest 

 waste. 



Properly speaking, all changes which rocks undergo after being 

 formed are metamorphic changes. According to this view, de- 

 cayed rock is a phase of metamorphic rock; but it has been cus- 

 tomary in the past to limit the term "metamorphic" to rocks which 

 are made more compact, more complex in constitution, or more 

 crystalline. Both sedimentary and igneous rocks may be meta- 

 morphosed. 



Induration of sediments. The first step in the alteration of 

 sediments is their induration, through the aid of cement, pressure, 

 etc. Sandstone and shale are not commonly called metamorphic 

 rocks, but they are metamorphosed sand and mud, respectively. 

 The cementing material of sediment, as already noted, is mineral 

 matter deposited from solution in water. Thus mineral matter 

 dissolved at and near the surface may be carried down by descending 

 water, and deposited between the grains of sediment, binding them 

 ti'izt ther. The cementation may be slight, or it may go so far that 

 all the spaces between the grains of sediment are filled. When the 

 spaces between sand grains are filled with silica, the rock becomes 

 quartzite. Between loose sand at the one extreme, and quartzite 

 at the other, there are all gradations. Quartzite is classed as meta- 

 morphic rock, but it is formed by a continuation of the process which 

 converts sand into sandstone. Important changes in rock are 



