igoo-i.] Physical Geology of Central Ontario. 157 



planation surface, and the residual monadnocks, as indicated by the 

 even-topped upland. This surface was uplifted to permit of the 

 renewed activities that carved and denuded the ungraded, or partly- 

 graded slopes of sub-maturity, now presented wherever it has been freed 

 from the Palaeozoic cover. This latter dissection and denudation ante- 

 dates the sediments, within this area, commonly called Potsdam, and 

 may well have taken place in early and middle Cambrian times. 



The ancient pre-sedimentary surface may be conveniently described 

 as a sub-maturely dissected and denuded peneplain dating from before 

 early Cambrian times. 



THE PALEOZOIC SERIES. 



A Qiiestio7i of Correlation. — In tracing the geological history of this 

 area, by means of the nature and relations of the different deposits 

 found adjacent to and within its boundaries, there is a question that 

 must not be disregarded, as to the correlation of partly eroded stratified 

 deposits, at a low angle of dip, around the margin of an oldland area. 

 In the formation of a series of deposits upon a slowly sinking land 

 surface, the normal distribution of material is the formation of sand and 

 pebble beds at the shore line, grading gradually into clays and muds, 

 and thence into calcareous deposits (figure 9). Any given stratum 

 must have three synchronous members, each merging gradually into the 



Sea Level 



Figure 9. — Diagram to represent the normal distribution of sediments. A, oldland ; B, sandstone and 

 conglomerate zone ; C, shale zone ; D, limestone zone. Transition zones are indicated by lines. 



adjacent member. The beds composed of strata which have been 

 deposited successively must also each consist of these three members. 

 During the time of th^ formation of any given bed the forms of life 

 existing at that time will be distributed over the surface of that bed, 

 each in its appropriate locality. The sand-loving forms will be near the 

 shore, the mud-loving forms in the areas which afterwards become 

 converted into shale, and the forms which thrive best in deep clear 

 water will be found further seaward. At the transition zones where 



