CHAPTER VIII. 



STRUCTURAL (GEOTECTONIC) GEOLOGY. 



The structural phases which rocks assume. — In the previous chap- 

 ters, the general method by which rocks are formed has been set forth, 

 and many of their structural features have been touched upon inci- 

 dentally. It remains to assemble the structural features already men- 

 tioned, and to consider certain additional structural phases which rocks 

 assume. 



STRUCTURAL FEATURES OF SEDIMENTARY ROCKS. 



In the deposition of sediments in the sea, or in other bodies of stand- 

 ing water, the coarser portion of the material is usually deposited in the 

 shallow water near the shore where the wave-action is strongest, and the 

 less coarse of various grades is deposited at greater and greater distances 

 from the land, while only extremely fine silt is usually carried out to 

 abysmal depths (see p. 380). To this general law of distribution 

 there are important exceptions. Fine sediments are sometimes deposited 

 near the shore, and where currents, tidal agitation, or floating ice are 

 effective, coarse deposits are occasionally carried far out from the shore. 



Stratification. — Sedimentary rocks are usually arranged in more or 

 less distinct layers; that is, they are stratified. The stratification con- 

 sists primarily in the superposition of layers of different constitution 

 or different compactness on one another. Layers of Hke constitution 

 or compactness are often separated by films of different material which 

 cause the partings between them. The bedded arrangement of strati- 

 fied rocks is due to various causes, but primarily to the varying agitation 

 of the waters in which the sediment was laid down. Where the depositing 

 waters are agitated to the bottom, coarse sediment is Hkely to be de- 

 posited. Where the waters are quiet at the bottom, fine sediment is the 

 rule. Since the agitation of the waters is subject to frequent change, 

 it follows that coarser material succeeds finer, and finer coarser, in the 

 same place. Hence arise beds, layers, and lamince. The terms layer 

 and bed are generally used as synonyms, while lamince are th'nner divi- 



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