374 EHREXFELD— JOINTIXG AS A FUNDAMENTAL FACTOR 



here certain physical conditions which are favorable to the preva- 

 lence of jointing influence. These are the marked mechanical na- 

 ture of the disintegration and the rapidity of deposit of the sedi- 

 ments before chemical weathering or disintegration had had time to 

 act, the repeated occurrences of coal swamps overrun by heavily 

 bedded gravels which indicates an unstable rock equilibrium fa- 

 vorable to the idea of the presence of joints; and finally the pres- 

 ence of subangular blocks and large pebbles and bowlders shows a 

 rapid mechanical disintegration. It would seem to be evident then 

 that the accumulation of conglomerates, such as in the Pottsville, 

 demand a rapid mechanical disintegration running ahead of chem- 

 ical weathering, and a grinding and transporting agent to reduce 

 the joint blocks to rounded bowlders, pebbles and sands before 

 chemical disintegration has time to occur. This, it will be observed, 

 is practically the same idea as shown in the formation of arkose 

 sediments generally. This point will be discussed later in con- 

 nection with the similar formations in the Newark formation. 

 This reducing agent referred to need not necessarily be marine. 

 In the small tributaries to the Monongahela river, small streams 

 flowing rapidly down slopes across jointed beds of sandstone, lime- 

 stone and shale, I have frequently collected rounded oblong pebbles 

 6 to 8 inches in length and also roughly rounded rectangular blocks 

 of limestone which are manifestly the result of surface erosion 

 and grinding on blocks due to jointing. The shales under these 

 conditions pass rapidly to muds and are borne along to make river 

 flats, alluvial plains upon which river and stream ice might deposit 

 the larger pebbles and even the angular fragments ; the ability of 

 river ice to do this is a matter of fact proven by observation. The 

 dry summer stages of such tributary streams show many such cases 

 of comparatively contemporary joint degradation; often the orig- 

 inal joint structure would have to be inferred from the nature of 

 the surface gravels if it could not be seen in the immediately adjoin- 

 ing hills. 



In the case of marine planation there is also the further fact 

 that an increase in the encroachment of the sea upon land masses 

 acts in a manner analogous to the rejuvenation of a stream by 

 elevation of the land surface, it increases its cutting and transport- 



