DIASTROPHISM AND THE FORMATIVE PROCESSES 517 



probably not strictly true and perhaps not approximately true in 

 some cases. On this assumption the topset deposits of the conti- 

 nental shelves were of the same order of vertical depth as those 

 recently formed. These we commonly agree to limit to the work 

 of the upper 100 fathoms of water, though the sum of the thick- 

 nesses of the topset strata is not Hmited to 100 fathoms. The 

 variable effects due to variable ocean depths are hence manifested 

 almost solely in the foreset beds formed on the terrace faces over- 

 looking the abysmal deeps. 



Now if we assume, for a concrete example, that the slope of 

 these foreset beds is i in 12, and that the mean depth of the abysmal 

 part of the ocean adjoining is 5,000 meters, a symmetrical stratum 

 of such an order as to advance the terrace horizontally one meter 

 oceanward will be 1/12 meter thick, and will have a radial width 

 of 60 kilometers. This stratum is equivalent to a vertical stratum 

 one meter wide and 5,000 meters deep. If any other slope than 

 I to 12 be taken as representative, a similar result follows; that 

 is, a stratum that measures one meter horizontally at the surface, 

 may be reduced to a vertical stratum one meter wide and 5,000 

 meters deep. The measure of horizontal advance and the measure 

 of ocean depth thus appear to be the true factors in measuring sedi- 

 mentary growth about the continents and to be conveniently simple 

 in use. The slopes and the stratigraphic thicknesses may in many 

 cases be neglected. They must of course be duly taken into account 

 in their own appropriate fields but these are not precisely this field. 



Notwithstanding the simplicity of the method thus deduced, 

 all determinations of the quantitative values of the sediments of the 

 circum-continental terraces are embarrassed by several serious 

 difficulties. In spite of these, however, some working concepts 

 may be formed that have value, even though they do not rise 

 above very rough approximations. 



In any specific attempt to deal with the sediments of the terrace 

 shelf it is of course necessary to select the epoch or the formation 

 from which the beginning of terrace growth is to be reckoned. It 

 is idle in the present state of knowledge to go back of the Cambrian 

 period and in many problems a much later date is best taken as the 

 starting-point. 



