DIASTROPHISM AND THE FORMATIVE PROCESSES 271 



much lower slopes occur. These slopes vary with conditions, as 

 in the case of the topset beds, but, in the nature of the case, they 

 persist at some grade so long as there is continental feeding and 

 effective shelf-sea action. 



If now the picture of the slopes of the topset and of the foreset 

 beds of the continental shelves, and their relations to one another, 

 are clearly in mind, and it is plainly seen that they are persistent 

 and systematic features, we are prepared for the question: What 

 is the limit of thickness of foreset and topset beds in an ocean of 

 given depth where no subsidence or creep takes place ? As thus 

 stated, no determinate answer to the question is possible. The 

 depth of the ocean — -beyond a sufficient working depth — ^is essen- 

 tially immaterial so far as the theoretical possibilities of thickness 

 are concerned. It is rather the breadth of the ocean than its depth 

 that controls the possible thickness of such sloping beds. The 

 growth of the continental shelves oceanward increases the thickness of 

 the topset and foreset strata irrespective of the ocean depth. The ocean 

 depth merely determines the amount of material required to effect 

 the oceanward growth. While it may thus influence the rate of 

 growth in thickness or the ratio of topset beds to foreset beds, or 

 their slopes, or other details incidental to thickness, it is not vital 

 to the possibilities of thickness. 



If all this is not evident from the mere statement of the case, 

 it should become so from an inspection of the accompanying figure 

 in which it is assumed that the crust remains absolutely stationary, 

 that there is a continual supply of material from the land, that the 

 sea surface changes only as it is forced to rise by sea-filling, that the 

 depth of effective shelf-sea action is 100 fathoms, and that a normal 

 distribution of sediment arises from the movements of the sea- 

 water. According to standard methods of finding the thickness of 

 beds, the measurement is taken on lines normal to the deposition 

 planes. This is obviously the correct method, as it sums up the 

 successive increments of sedimentation. It will be seen from the 

 diagram (Fig. i) that the topset and the foreset beds increase by the 

 addition of layer laid obliquely upon layer in the course of the 

 growth seaward and that these lie in essentially the same horizon 

 and may accumulate quite irrespective of the total depth of ocean. 



