THE PROTEROZOIC ERA 



463 



the bedding-planes, and the sum of the whole ascertained; or (2) 

 the average dip of the beds is taken, and the distance across the 

 truncated edges, eo, is measured, and the thickness of the whole, 

 ef, computed. By comparing the line ef, representing the thick- 

 ness of the series, with the line cd, the depth of the basin when 

 the beds were deposited, a marked difference is seen. Moreover, 

 the difference may vary so much that there is no necessary relation 

 between the two. If the line ef is short, as it would be if the series 

 had not been built far out, the thickness would be less than the 

 depth of the ocean, cd; but if the line ef is long, as it would be 

 if deposition were continued sufficiently long, the thickness of the 

 series, would be proportionately increased, while cd might re- 

 main constant. In other words, the thickness of the series may 

 vary from any fraction of the depth of the basin to a large multiple of it. 

 Similar considerations reveal a discrepancy between the vertical 

 measurement and the thickness of the beds deposited subaerially. 

 For example, the thickness of the beds of a volcanic cone is less 

 than the height of the cone which they form, as will be seen from Fig. 

 354. Lava-flows which congeal as they spread / may give rise to a 



Fig. 354. Diagrammatic illustration of relation of thickness of beds to 

 height of accumulation in the case of volcanic cones, ac, height of 

 cone; be, aggregate thickness of layers, which is obviously less than ac. 



series of great thickness, without implying contemporaneous sink- 

 ing. Clastic beds formed by slope-wash may be laid down between 

 the lava-flows without implying subsidence. In the case of the 

 Keweenawan system, a congelation or deposition slope of 5, ex- 

 tended horizontally a little more than one hundred miles (about 

 half across the Keweenawan basin as it may have been) would 



