154 Transactions of the Canadian Institute. [Vol. VII. 



the conditions of the pre-Cambrian floor, and upon the distribution of 

 the Cambrian sediments over North America, is that during the interval 

 of the deposition of early Cambrian sediments there was a great interior 

 continent, of which Central Ontario would form a part. 



Walcott ('91, 567) thus sums up the conclusions from his studies on 

 "North America during Cambrian time": — i. "The pre-Cambrian 

 Algonkian continent was formed of the crystalline rocks of the Archean 

 nuclei, and broad areas of superjacent Algonkian rocks that were more 

 or less disturbed and extensively eroded in pre-Cambian time. Its area 

 was larger than at any succeeding epoch until Mesozoic time." 

 ********** * *^ 



4. "The interior continental area was, at the beginning of Cambrian 

 time, an elevated, broad, relatively level plateau between the Paleo- 

 Appalachian sea on the east, and the Paleo-Rocky Mountain barrier on 

 the west." 

 ******** ***** 



7. "The Cambrian Sea began to invade the great Interior Contin- 

 ental area in late Middle Cambrian time, and extended far to the north 

 toward the close of the period." 



8. " The depression of the continent in relation to sea level began in 

 pre-Cambrian time and continued with a few interruptions until the close 

 of Paleozoic time." 



Many conclusions with reference to events which occurred so long 

 ago must necessarily be somewhat uncertain. With our present know- 

 ledge of the evidences, it seems that during Archean and early Cambrian 

 times this area formed part of a continental area. The processes by 

 which the even-topped upland was produced operated so long ago that 

 it is impossible to determine their precise nature. The character of the 

 subsequent dissection appears to indicate that the present topographic 

 features of the uplands were, in the main, the product of subaerial 

 erosion during a pre-Potsdam period of elevation. 



The balance of evidence thus leads to the conclusion that the present 

 surface features of the crystalline area, at least along the borders of the 

 Palaeozoic sediments, are essentially the same as they were in pre- 

 sedimentary times. The problem now arises as to the process by which 

 the degradation and the denudation produced the even- topped upland 

 and the varx'ing features of the present topography. 



It would be well to note with reference to the term even-topped, that 

 a personal equation must be considered. The expression is used here 



