THE CAMBRIAN PERIOD. 



259 



the slope, the deposits have doubtless varied much in character. They 

 were doubtless laid down fastest, and were coarsest, when the shore 

 stood on the slope, as it has recently done, and as it probably has usually 

 done after periods of body deformation when the seas were withdrawn 

 more fully than usual within the oceanic basins, and when the conti- 

 nents were hence more protuberant and gradation more active. On 

 the other hand, when the thin edge of the sea lapped over the border 

 of the continent, the deposition on the abysmal slope was doubtless 

 slower and the deposits were composed of finer material, forming shale 

 or limestone. These details are merely to emphasize the persistent 

 nature of the deposition on the abysmal slopes, and its varied character. 

 The upper portion of the deposit on the abysmal slopes took place 

 normally in rather shallow water, and from its exposed position this 

 upper portion must always have been especially subject to the action 

 of ocean currents and tidal waves which tended to give it the character- 

 istics of agitated water-deposits, which we usually interpret as shallow 

 water deposits. 



The abysmal slope usually dips from 2° to 5° and hence these are 

 about the deposition angles of the beds laid down on it. Now when 



Fig. 108.— Diagram of a series of Ixds formed on the abysmal slope of a continent, 

 or in similar situations, showing that the thickness as usually measured is not 

 dependent on the depth of the basin, 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. A, border of the continent, or rim of the 

 basin; B, border of the depositional area; C, ocean-level; D, ocean-bottom previous 

 to deposition; S, slope of abysmal basin at the beginning of deposition; EF, sur- 

 face developed by erosion after emergence; L, beds deposited in sloping attitude; 

 EFG, triangle by which the thickness of the series is measured; FG, computed 

 thickness of beds. When compared with CD, the depth of the basin, a marked 

 difference will be seen, and it will be obvious that the difference will vary with 

 the length of EF. 



these beds have become accessible by deformation and truncated by 

 erosion, they are measured along the surface, which may be represented 

 by the line EF, Fig. 108. There are two common modes of measurement : 

 (1) The beds are measured individually, or by groups, at right angles 

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



