1/2 A. C. LAWSON — I'll I. PRE-PALEOZOIC SI RFAC1 



deposition of the Niagara than in earlier epochs. It would follow from 

 these considerations, that as Paleozoic time advanced from Cambrian to late 

 Silurian or Devonian there was a gradual and progressive subsidence of this 

 portion of the continent. A- we have no evidence of the deposition of post- 

 Devonian formations anywhere over the Archean "nucleus" till we come 

 down to post-Tertiary, it may In- tentatively inferred that after the Devonian 

 it was again elevated, and this elevation probably only reached its maximum 

 during the glacial epoch, affording the conditions of altitude contended for 

 by many writers to explain the great precipitation of -now. In post-glacial 

 times we kmiu from the distribution of such formations a- the Leda clay 

 and Saxicava -ami that the northern part of the continent was again par- 

 tially submerged for several hundred feet, from which depression it has since 

 recovered : we thus have evidence of a slow ve tical pulsation of the surface 

 of this part of the continent, of which there have been at least four great 

 beats since early Cambrian time.-. 



But this is a digression, and the argument which has led to these remarks 

 wa- inaugurated to .-how simply that the surface of the Archean •• nucleus " 

 wa- once very extensively if not wholly covered by Paleozoic sediments. 

 This covering probably accounts in a large measure for the remarkable 

 preservation of the Archean surface in the condition in which pre-Paleozoic 

 denudation left it. There are other considerations which help us to under- 

 stand tin.- preservation, such as the levelness of the plateau and its corapari- 

 tively low altitude, combined with the very resistant character of most of 

 it- rocks, which appeal- to lie little susceptible to that erosive or corrasive 

 action of streams which i- so effective in removing the more yielding Btrata 

 of post-Archean age. The-, considerations will not, however, he em, Ted 

 upon hi 



'I'ln I m "(Hi' Archean. — One i- constantly impressed by the perfectly 

 appalling amounl of denudation t<> which the Archean has been subjected in 

 order to truncate its formation- down to the surface which it presents to-day. 

 A ml w hen «re reflect, as a result of the conclusions here arrived at. that this 

 denudation was practically completed before the beginning of earliest 

 Paleozoic time-, ami has not been, a.- commonly supposed, the result of later 

 there looms up a conception of the pre-Paleozoic interval necessary 

 for Buch denudation which stag ten the most Btalwart geological 



imagination. T" saj that i; must have been comparable with all the time 

 which ha- succeeded from the earliest Cambrian to the presenl seems but a 

 feeble way of exp it. 



P Sediments. — The c iption of a covering of Paleozoic 



strata over the gurfai I the Archean " nucleus," which probably endured 



into comparatively recent geological times, enables us to a larg< extent to 

 understand i I rvatiou of the pre-Paleozoic Burface, hut it also raises the 



