GEOSYNCLIXES 351 



shown by uplift and dei^ression of the crust. Fourth, the existence of 

 peneplains, or surfaces of erosion produced during a long period of quiet 

 on the land, disproves the idea that the land is continually in flux and 

 completely responsive to isostasy. Fifth, the mechanism appealed to for 

 a shift of the vertical force of gravity into horizontal motion is a very 

 deep-seated underflow of rock from sea to land. This is a wholly hypo- 

 thetical arrangement and must remain so. All that our observation tells 

 us is that lateral folding is most pronounced in the great piles of sedi- 

 ment, which a're seldom more than five miles deep and in a few miles of 

 closely underlying granite and gneiss. Sixth, the differences in density 

 brought out by pendulum and plumb-line observations have yet to 

 undergo careful and detailed comparison with the facts of geology. 

 Seventh, the sediments whose transfer initiates isostasy are only thick 

 near the sea margin; their maximum thickness is rarely over five miles 

 and their average is a mile or less for the area of deposition concerned. 

 This load is supposed to overpower the crust and to cause huge transfers 

 of material ; in the case of the Appalachians this transfer is of the order 

 of 4,000,000 cubic miles. Eighth, the thick sediments whose weight is 

 supposed to depress the sea-floor and cause the underflow are the very 

 masses which are folded and raised the highest; this is truly an achieve- 

 ment in dynamics. 



In the matters Just mentioned there is a serious lack of agreement be- 

 tween theory and observation. It is therefore logical to conclude that 

 isostasy is not the cause of folded mountain structure. That conclusion 

 does not, however, deny or even cast doubt on its action in a minor way 

 to reduce and perhaps remove inequalities produced by the action of 

 major forces. 



GE08YNCLINES 



It has been said by some writers that the accumulation of sediment in 

 geosynclines was the cause of folded mountain structure. This, how- 

 ever, in so far as the association of folding and geosynclines is true, is 

 merely one step in the line of causation. The thick deposits in the syn- 

 clines merely serve by their easy folding to localize the deformation. 

 Far back of that lies another cause which produced the geosynclines 

 themselves. 



As has been shown, the geosynclines and the strongly folded belt are 

 only coincident in a general way in the Appalachians, and the result in 

 the location of the folded belt would have been practically the same, 

 even if the great mass of Carboniferous deposits in the geosynclines had 

 not been laid down. The geosyncline as shown in the Carboniferous 



