396 EHRENFELD— JOINTING AS A FUNDAMENTAL FACTOR 



uplifted or folded areas, may be imagined to pass into a zone of 

 fracture by a lessening of the distance between the outer atmos- 

 pheric contact and the mass of rock in question. I do not wish to 

 complicate the discussion by citing hypothetical cases, but we may 

 perhaps legitimately imagine that folded areas such as the Appa- 

 lachians and the Alps would continue to be subject to earth strains 

 after erosion had removed a considerable part of the overlying 

 weight of rock mass. This would then allow these folded masses to 

 pass into fractured masses of various degrees of intensity. The 

 bearing of this on joint control of degradation is, I think, obvious 

 enough. 



In connection with other courses of the rock mass of the earth 

 which are not subject apparently to these intense pressures, such 

 as the Canadian Shield, I have already stated that I believe the 

 source of fracture there to be the successions of overloading by sedi- 

 mentation, ice accumulation, marine inundation, with the consequent 

 wearing out of the elasticity of the original rock mass. The rock 

 mass under these circumstances would then simply proceed to fall 

 apart, depending upon the presence of some agent such as marine 

 erosion capable of removing the blocks due to jointing. The illus- 

 trations given of the disintegration of the mass of the coastal plain 

 near Ogunquit, Me. (Figs. 5-6. Plate IX) will show practically how 

 a mass of folded rocks will when exposed to release of pressure break 

 down into segments which can easily be seen to be the result of 

 wide reaching joints. Almost the whole coast of IMaine is an illus- 

 tration of this. 



As for these rocks near Ogunquit, they show all the marks of 

 what is usually referred to as rock bending under pressure, S-folds 

 are not uncommon, isoclinal folds are evidently quite frequent and 

 it is a very probable supposition that here is a mass of rock w-hich 

 has been under such stress that distinct folds developed and that 

 now very definite lines of jointing are in control of the lowering 

 of the land mass. 



It is of course entirely possible or even probable that some of 

 this jointing was simultaneous with the folding. I hope in a later 

 paper to discuss the evidences for this. 



But the essential fact stands out that here is a portion of the 



