CHAPTER XI 

 STRUCTURAL (GEOTECTONIC) GEOLOGY 



The general method by which rocks are formed has been set 

 forth in previous chapters, and many of their structural featmvs 

 have been touched upon. It remains to assemble the structural 

 features already mentioned and to note certain additional ones. 



Sedimentary Rocks 



Stratification. Sedimentary rocks are usually arranged in more 

 or less distinct layers; that is, they are stratified. Stratification 

 consists primarily in the superposition of layers of different con- 

 stitution or compactness, on one another; but layers of like consti- 

 tution or compactness are often separated by films of different 

 material which cause the partings between them. The bedded 

 arrangement of stratified rocks is due to various causes, but pri- 

 marily to the varying agitation of the waters in which the sediment > 

 were laid down. Where the depositing waters are agitated to the 

 bottom, coarse sediment only is likely to be deposited. Where 

 the waters are quiet at the bottom, fine sediment is the rule. 

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

 coarser material may succeed finer, and vice versa, in the same 

 place. Hence arise beds, layers, and lamince. The terms layrr 

 and bed are generally used as synonyms, while lamince are thinner 

 divisions of the same sort. The term stratum is sometimes applied 

 to one layer, and sometimes to all the consecutive layers of the same 

 sort of rock. For the latter meaning the term formation is often 

 used. 



In some cases bedding seems to have been determined by ^t rou^ 

 currents which temporarily not only, prevented deposition over 

 a ^i ven area, but even cut away the loose sin-face of deposit-; already 

 made, s'vins Ji firm surface from which succcedi:^ deposits are 



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