Vice-President's Address. 170 



yet none of these limestones is of abysmal origin. They 

 prove that portions of the continental plateau have now and 

 again been submerged for several thousand feet, but afford 

 no evidence of depths comparable to those of the present 

 oceanic basins. The enormous thickness attained by the 

 sedimentary strata can only be explained on the supposition 

 that deposition took place over a gradually sinking area. 

 And thus it can be shown that, within the continental 

 plateau, movements of depression have been carried on more 

 or less continuously during vast periods of time — and yet so 

 gradually, that sedimentation was able to keep pace with 

 them. Take, for example, the Cambrian strata of Wales and 

 Shropshire — all, apparently, shallow water deposits — which 

 attain a thickness of 30,000 feet, or thereabout ; or the 

 Silurian strata of the same regions, which are not much less 

 than 20,000 feet thick ; and similar great depths of sedi- 

 mentary rocks might be cited from North America. Passing 

 on to later periods, we find like evidence of long-continued 

 depression in the thick sediments of the younger Palaeozoic 

 systems. It is noteworthy, however, that when we come 

 down to still later ages, the movements of depression, as 

 measured by the depths of the strata, appear to have become 

 less and less extensive and profound. Each such movement 

 of depression was eventually brought to a close by one or 

 more movements of upheaval — slowly or more rapidly 

 effected, as the case may have been. Here, then, we are 

 confronted with the striking fact that the continental plateau 

 has, from time to time, sunk down over wide areas to depths 

 exceeding those of existing oceans, and yet at so slow a rate, 

 that sedimentation prevented the depressed regions from 

 becoming abysmal. It is obvious, then, that such areas are 

 now dry land simply because, in the long run, sedimentation 

 and upheaval have been in excess of depression. 



And yet, notwithstanding the numerous upheavals which 

 have taken place over the continental plateau, these have 

 succeeded in doing little more than drain away the sea more 

 or less completely from the great primeval depressions by 

 which that plateau is traversed. If it be true, therefore, that 

 the continental plateau owes its existence to the sinking 



