IN THE DEGRADATION OF THE LITHOSPHERE. 375 



ing power and also increases the rate of land degradation. Thus 

 the factor of jointing serves to unite processes of atmospheric and 

 marine degradation and because it has probably done so in the past 

 as well, it enters into the problem of paleogeography. The trans- 

 gressions of the sea, the filling of old sea channels and bays by 

 masses of jointed limestone, sandstone, or the clay rocks, material 

 which would rather readily reduce to sands and gravels and muds, 

 all become factors which may eventually perhaps be shown to be 

 due to jointing degradation. The interbedded limestone conglom- 

 erates observed by Logan, Prime, Walcott^- and others certainly 

 suggest a method of origin involving both a mechanical method of 

 rapid erosion and also a time interval between the deposition of 

 the original limestone layers and their subsequent breakdown to 

 gravel or even bowlder form. Anyone who has personally exam- 

 ined these conglomerates in York County, Pa., can have no doubt 

 as to the angular nature of the fragments of the conglomerate and 

 Walcott (p. 39, op. cit.) refers to the "angular fragments of lime- 

 stone with sharp, clear cut edges " in the redeposited conglom- 

 erates ; in some cases these bowlders are " from 3 to 4 feet " in 

 diameter, these last from Tennessee. In discussing the origin of 

 these conglomerates Walcott believes " that the sea bed was raised 

 in ridges or domes above the sea level and thus subjected to the 

 action of sea shore ice, if present, and the aerial agents of erosion. 

 . . . The inference is drawn that the debris worn from the ridges was 

 deposited in the intervening depression beneath the sea." Walcott 

 does not consider jointing in this connection at all though it may 

 be regarded as a necessary adjunct for more than one reason. For 

 one thing it is very much to be doubted if marine erosion as hy- 

 pothecated could occur unless the rock masses of the limestone were 

 already in some separated block form, ready to be torn apart and 

 beaten about by tidal action ; if on the other hand erosion against 

 a sea cliff or against a mass of rock exposed to atmospheric weath- 

 ering be regarded as having occurred we must again, in the case 

 of limestone, believe that jointing was the predominant factor as 

 otherwise the limestone would be reduced to weathered residues 



22 Bull. U. S. G. S. No. 134, PP- 34-40; pis. X.-XIV. Especially p. 36 

 and p. 39. 



