EVIDENCE AS TO AGE 585 



coast southwest of New Orleans between 11,000 and 15,000 feet, the base 

 being- far lower than the continental shelf and one-half to three-fourths 

 as low as the bottom of the Gulf. 



NATURE OF DERIVED DEPOSITS 



The use of fineness of grain and chemical deposits in dating peneplains 

 seems rather unsafe, because it so generally depends on which peneplain 

 and which limestone are to be correlated, and there seems to be much dif- 

 ference of opinion as to the length of erosion cycles. Does the develop- 

 ment of a peneplain generally require periods or epochs, or less than an 

 epoch, or is it quite impossible to estimate ? If it were known that about 

 a period is required, that there are in the Appalachian province but two 

 or three peneplains, and in the deposits two or three main limestones 

 separated by many formations, the presumable implication would be clear. 



AMOUNT AND RATE OF EROSION 



If 30 million years have elapsed since Paleozoic time and the rate of 

 erosion of the province has been the same as the present average rate for 

 the United States, an average of over 3,000 feet of rock would have been 

 removed, and it would seem ^ery unlikely that any portion of a surface 

 formed in pre-Cretaceous time could have survived this general degrada- 

 tion. To be sure, this figure may be far from correct, but it seems doubt- 

 ful if it is many fold too large. So surely as rain has fallen on the earth, 

 every square mile of the land area has annually lost through solution 

 alone a great many tons. Furthermore, the streams of lowlands, com- 

 monly at least, carry much sediment ; so that, whether high or low, no part 

 of the Appalachian province except those areas where deposits have been 

 formed can have escaped continual reduction. 



If the Cretaceous and Cenozoic deposits of the Coastal Plain w^ere to be 

 spread evenly over the area outside the Coastal Plain now draining to the 

 Atlantic and eastern Gulf, they would apparently make a layer about 

 3,200 feet thick, and probably the reduction that they represent is con- 

 siderably more, one reason being that much of the lime, etcetera, of the 

 eroded area has been carried far away. True, the Coastal Plain border 

 has been shifting seaward ; the eastern border of the Mississippi basin has 

 migrated more or less, and some of the Coastal Plain sediments may have 

 been brought along shore from other regions; but after allowances are 

 made, it seems probable that the quantity of Coastal Plain sediments indi- 

 cate a removal from the Appalachian region of an average thickness be- 

 tween 2,000 and 5,000 feet. If tlie amount were only 1,000 feet, it would 

 still be sufficient for the present argument. 



