262 



GEOLOGY. 



the thickness of the series on each side, measured by the above stan- 

 dard method, would be 13,800 feet, no change at all in the bottom or 

 the surface of the lake being assumed. If the Nile should fill up the 

 eastern end of the Mediterranean, the Danube the Black sea, and the 

 Mississippi the Gulf of Mexico, with no essential change in the bottoms 

 of the basins or the surfaces of the water-bodies, series of beds of pro- 

 digious thicknesses, as thicknesses are commonly estimated, would be 

 the result. The usual changes in the sea-bottoms, changes of both 

 upward and downward phase, might take place during the process of 

 filling, without seriously affecting the result. 



Fig. 110. — Diagrammatic section illustrating the assigned change of attitude of a 

 series of beds, like the Keweenawan, from an original depositional inclination, 

 to a more highly inclined attitude, a comparatively simple change. If the beds 

 were laid down horizontally in a sinking basin, as illustrated at the right, it is 

 obvious that a greater and a more complicated movement would be necessary to 

 bring the beds into the attitude represented in the lower figure at the left, which 

 represents the present attitude of the Keweenawan beds. 



Dwelling on these aspects of the case, we almost reach the con- 

 clusion that the thickness of a series of beds, as we usually measure 

 thicknesses where they are great, is independent of the depth of the basin. 

 This, howcvgr, is pushing the case too far, for the considerations that 

 have been urged apply only to deposition of the kinds specified, i.e., 

 deposition on appreciable slopes. When deposition takes place by 

 strict superposition in horizontal attitudes, the usual inference of a 

 correspondence between the depth of the basin, or the sinking of its 

 bottom, and the thickness of the series, holds good. Moreover, when 

 the beds above and below are accessible, or penetrable by borings 

 such as wells, over large areas, the vertical depth of deposition may be 

 conclusively ascertained. This is usually the case with the deposits 

 of the broad shallow epicontinental seas. In them, oblique deposi- 

 tion is usually a minor phenomenon. But the deposits of the shallow 

 epicontinental seas, laid down on plane surfaces, should not be confused 

 with the deposits built out on the border slopes of the deep basins. 



