EXTENT OF PALEOZOIC CONTINENTAL SEAS 



313 



area. What I do wish to say is that this erosion was practically always 

 confined to the periodically rejuvenated upwarped areas, and that the 

 marine or lake deposits were similarly confined to and laid down over 

 each other in the same more or less limited superposed basins. With 

 each of the successive upwarps the beds which were first deposited on 

 their flanks become more and more inclined, and as the older formations 

 are rarely seen except near the axis of the arch the contacts naturally are 

 more distinctly unconformable here than farther out in the basins. Obvi- 

 ously, again, the more extreme the uplifts the less time is required to cut 

 through the upturned and relatively thin overlapping edges of the forma- 

 tions. Besides, other factors being equal, erosion is facilitated in increas- 

 ing ratio to the amount of relief. Hence in estimating the time value of 

 a gap in the record we are not justified in concluding that because an 

 overlying conglomerate contains pebbles derived from formations aggre- 

 gating, say, 10,000 or 20,000 feet in thickness, the gap represents a time 

 interval of sufficient length to remove a corresponding thickness of de- 

 posits. Statements implying erosion of such magnitude have been fre- 

 quently made in discussions of the Laramie problem — as, for instance, 

 by Cross — ^who in one of his papers says, "In the pebbles of the Arapahoe 

 ... is the record of the slow erosion of 14,000 feet of strata." ^ 



What amounts actually were removed from the "positive" areas we 

 have no satisfactory means of determining. When no remnants are pre- 

 served, apparently the only clue lies in the thickness and lateral extent of 

 the deposits which they contributed to adjacent "negative" areas. A 

 point that should be made in this connection is that whereas the proposi- 

 tion of permanence of positive and negative areas, together with the 

 hypothesis of small, oscillating continental seas, reduces the hitherto 

 assumed total amount of erosion, it reduces also the amounts required to 

 fill the limited basins. Besides, we know well that only a small propor- 

 tion of the clastic sediments extends far beyond the heavily laden troughs 

 immediately adjacent to areas of folded and contorted pre-Cambrian 

 rocks, which, after all, contributed by far the greater amount of such 

 material. 



PERIODIC REVIVALS OF EROSION PROCESSES 



Assuming that baseleveling was in progress through all the ages, it vet 

 seems that at certain recognizable times the process was strongly revived. 

 Local accelerations and revivals, especially in the submarginal areas of 

 the continent, occurred at frequent and not easily classified intervals. 

 But the greater revivals, implying deformative movements of l-)roacler. 



* W. Cross: Post-Laramie deposits of Colorado. Am. Jour. Sci., vol. xllv. July. 1892. 

 XXII— Bull. Geol. Soc. Am., Vol. 32, 1910 



