THE CAMBRIAN PERIOD. 



261 



parent thickness of the Keweenawan series, certainly not that it is 

 the whole explanation, though 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 havo an insecure basis. 



In the case of such sedimentary beds as are formed on mountain 

 slopes, the sides of intermontane valleys, or in other situations where 



Fig. 109a. 

 Figs. 109 and 109a. — Diagrammatic illustration of relation of thickness of beds to 

 height of accumulation in the case of volcanic cones. AB, height of cone; CB t 

 aggiegate thickness of layers, which is obviously less than AB. 



there is an appreciable deposition angle, the thickness of the beds varies 

 indefinitely in comparison with the vertical range of the deposit. 



Delta deposits furnish especially good illustrations of the point 

 under discussion, and delta deposits have probably been elements in 

 the deposits of all the great periods, though not always readily recog- 

 nizable in their buried condition. If the Amazon were to build out a 

 delta 200 miles, the ocean bottom remaining absolutely immovable 

 at an average depth of four miles below the surface, and the deposition 

 angle were to be on the average 2°, the computed thickness of the series, 

 according to current methods of measurement, would be about seven 

 miles. If the delta were built out 1000 miles, the computed depth 

 would be thirty-five miles. In a lake 100 miles wide and 1000 feet deep, 

 in wliich the deposition angle was 3° on the average, and in which the 

 filling grew outward from each shore fifty miles, meeting in the center, 



