THE LA URENTIA N PENEPLA IN 657 



interval of sufficient duration to permit of the foundation of a 

 plain of marine planation of any considerable breadth, the land 

 of a very much larger area would be reduced almost to base- 

 level by subaerial processes. If so, temporarily assuming that 

 the margin of the peneplain, from which the Paleozoic sediments 

 are known to have been removed, owes its plain character to marine 

 planation, it would follow that the interior portions were princi- 

 pally of subaerial origin. If the marginal portion of the plain 

 owed its plain character to marine erosion, we should expect to 

 find the debris removed from the ridges deposited in the hollows, 

 since the waves and currents tend always to produce an evenly 

 graded floor. In the field no trace of this can be found. The 

 Ordovician limestones often rest directly upon the plain surface 

 and pass down into the adjacent hollows where the sides are not 

 very steep. Where they are steep the lower beds abut against 

 the valley walls and are conformably overlaid by beds which 

 rest upon the plain. Nowhere has any accumulation which can 

 be regarded as distinctively a product of marine planation been 

 reported. Fossils (corals, crinoids, orthoceratites) are found in 

 the lower limestone beds sometimes within a few inches of the 

 Archaean rock, in some few cases actually attached to it. From 

 the fact that these are not comminuted it may be inferred that 

 the conditions of transgression, at least in the localities where 

 they occur were such that the waters were moderately quiet ; 

 and from the absence of arenaceous deposits in many of the 

 localities it may be concluded that the rate of transgression was 

 comparatively rapid — presumably too rapid too permit of sig- 

 nificant marine planation. 



That the rate of depression was probably too rapid for signifi- 

 cant marine planation is also suggested by the fact that everywhere 

 beneath the surface of the sediments the Archaean rocks are 

 found in a perfectly fresh and undecayed condition. 



Thus although the area must have been exposed to marine 

 planation at the time of Ordovician submergence there is reason 

 to believe that the conditions of submergence were such that 

 the pre-existing surface which must have been of subaerial origin 

 can have been but little modified. The distribution of overlying 



