FLEXURES AND FRACTURES OF THE EARTH'S CRUST. 817 



of the crust, causing oscillations of level ; (2) Fractures ; (3) the Evolv- 

 ing of Mountains and the attendant phenomena ; and, (4) as a final 

 result, the Evolution of the Earth's fundamental features. 



1. Flexures and Fractures. 



Flexure implies both upward and downward bendings, geanticlinal 

 and geosynclinal, the one a complement to the other. Such bendings 

 in the crust may have had a breadth of hundreds of miles, but not of 

 one mile, or twenty. If pushed into vertical folds, — that is, folds hav- 

 ing the opposite sides parallel, like many in the earth's strata, suppos- 

 ing this a possibility, — the breadth could not be less than twice the 

 thickness of the crust, which would be 400 miles for a crust 200 miles 

 thick, or 50 for a thickness of 25 miles. 



Such flexures are little like those of mountain regions ; for these 

 are foldings of the supercrust, and have seldom a span of thirty miles ; 

 rarely even twenty miles, and generally between one mile and ten, 

 though often less than one. The wrinkles of the contracting globe 

 have been compared to those over the surface of a drying apple ; they 

 are related in a general way as to cause ; but otherwise, the compari- 

 son fails. 



The flexures of the earth's crust were, for the most part, oscillations 

 in level of but a few scores or hundreds of feet, the upward move- 

 ment being often followed by as slight a reverse movement. On page 

 783 the fact that such oscillations occurred through all the successive 

 geological periods is illustrated by a reference to the Coal-formation. 



By causing variations of depth in the continental seas, such oscilla- 

 tions have led to variations in the flow, volume, and positions of cur- 

 rents ; in the kind and the quantity, therefore, of detritus which the 

 waters carried, or their freedom from any ; in the positions of coast 

 lines and of areas of marine depositions ; and thus they have deter- 

 mined variations also in the succession of rocks from sandstone to 

 shale, or conglomerate, or limestone, or the reverse, and in the areas 

 they were made to cover, such as geologists have found in all the for- 

 mations. 



The axis of these oscillations must have been at right angles to the 

 directions of pressure.. The directions which the conditions of the 

 earth's crust were calculated to produce were manifested, as has been 

 stated above, in the production of the outlines and mountain courses 

 of the Archaean land ; and these — which it is not necessary here to 

 review — were also, therefore, the later courses. They were not nec- 

 essarily straight lines, since the earth's surface would naturally have 

 had its large elliptical areas acting more or less independently. 



A second result from the force at work was the determining of 

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