464 



GEOLOGY 



give the estimated thickness. To explain the present attitude of 

 the beds, it is necessary to suppose that the basin was compressed 

 laterally, so that the beds were upturned and somewhat sheared 

 upon one another (Fig. 355). 



It is not here affirmed that this is the whole of the explanation 

 of the great apparent thickness of the Keweenawan system, though 





Fig. 355. Diagrammatic section illustrating the assigned change of atti- 

 tude of a series of beds, like the Keweenawan, from an original deposi- 

 tional inclination, A, to a more highly inclined attitude, B, a compar- 

 atively simple change. 



it seems to throw important light on it. The point here made is 

 merely that great apparent thickness may and does arise in this 

 way, and that inferences and doctrines that overlook this fact have 

 an insecure basis. 



Delta deposits furnish especially good illustrations of the point 

 under discussion. If the Amazon were to build a delta out 200 

 miles, the ocean bottom remaining immovable at an average depth 

 of four miles below the surface, and if the angle of deposition were 

 2, the computed thickness of the series, according to current 

 methods of measurement, would be about 7 miles. If the delta 

 were built out 1,000 miles, the computed depth would be 35 miles. 

 If a lake basin 100 miles wide and 1,000 feet deep were filled by 

 sediment, the angle of deposition from either side being 3, and the 

 filling from each side out to the center, the thickness of the series 

 on each side, measured by the above method, would be 13,800 feet. 

 In this case 13,800 feet of sediment would accumulate in a basin 

 1,000 feet deep. 



