462 



GEOLOGY 



crustal deformations have been built upon the supposed subsidence 

 of the surface. It is therefore important to inquire whether the 

 thickness of a series of sedimentary beds is a measure of the depth 

 of the basin in which they were deposited, or of its subsidence while 

 they were being laid down. 



When the sea covers the continental shelf, deposition takes 

 place both on it and on the slope to the abysmal depths of the ocean, 



Fig. 353. Diagram of a series of beds formed on the abysmal slope of a 

 continent, or in some similar situation, showing that the thickness, as 

 usually measured, ef, is not dependent on the depth of the basin, cd, and 

 that a thick series does not necessarily imply subsidence, even when 

 the exposed portions of it show evidences of shallow-water deposition 

 at various horizons. 



and to a slight extent on the bottom of the deep sea. When the 

 sea is withdrawn from the surface of the continental shelf, the shore 

 stands near the upper edge of the abysmal slope, and the heaviest 

 deposition is on this slope. The result may be seen from Fig. 353. 

 Sediment from the land, a, is carried to the sea and deposited 

 on the steep slope. The upper part of the deposits on the steep 

 slope takes place normally in shallow water, and from its exposed 

 position must be subject to the action of currents and waves 

 which give it the characteristics of deposits made in agitated 

 water. These characteristics are interpreted, commonly, as the 

 marks of shallow water. 



The abysmal slope is usually 2 to 5, and these are about the 

 angles of dip of the beds laid down on it. Now when these beds 

 have become accessible by deformation and erosion, they are meas- 

 ured along the surface, which may be represented by the line eo, 

 Fig. 353. There are two common modes of measurement: (1) The 

 beds are measured individually, or by groups, at right angles to 



