378 EHREXFELD— JOINTING AS A FUNDAMENTAL FACTOR 



The closing stages of the Permian have been supposed to show 

 even in North America evidences of glaciation. This may have 

 been sufficient to explain some of the arkose sandstones but there 

 hardly seems to be enough evidence at hand to hypothecate this as a 

 general condition extending into the Newark. All that I am en- 

 deavoring to demonstrate is the fact of a physical condition in the 

 surface portion of the rock structure favorable to easy and rapid 

 disintegration rather than to try to establish any one particular 

 means of the transportation of the broken rock fragments. 



On the edges of the Canadian shield or "peneplain" as it has 

 been called and along the edges of the continent bordering the Gulf 

 of Maine may be seen in active operation to-day the results of a 

 jointing structure to hasten marine erosion; this is taking place now 

 after glaciation has removed the surface soil cap, the jointing is 

 now fully exposed to whatever means of rock transportation may 

 be at hand. 



From the fact that practically all stages in the formation of 

 granite sand, pebble and bowlder flats may be observed in progress 

 along this coast we may reasonably infer that the formations out 

 beneath the low tide level extending over the continental shelf would 

 show the marked characteristics of a marine arkose analogous to 

 the continental arkoses of the Newark. There would thus seem to 

 be recurrent eras of a rapid joint degradation differing from the 

 longer periods of the usual chemical weathering and erosion, these 

 periods following either periods of earth strains or periods in 

 which after removal of the soil cap jointing would then proceed to 

 a further reduction of the lithosphere. 



Jointing, then, acts as a connecting factor in uniting continental 

 and marine erosion into a process or series of processes which, so 

 far as the lithosphere is concerned, are consecutive. Here again 

 the real controlling factor in the degradation is the structure of the 

 lithosphere itself, not the particular agent, because, conceivably, 

 surface continental erosion followed by marine erosion are simply 

 two stages of a general degradational process. There must have 

 been in former geological times reduction of the land surfaces after 

 the completion of a cycle of erosion ; it does not follow that every 



