2 INTRODUCTION. 



while not one of the denudants could act efficiently 

 without the aid of external or internal heat to dry 

 and contract the rocks; because to the drying and 

 contraction of the surface rocks is due the minor 

 cracks and joints; and only in conjunction with these 

 shrinkage-fissures can any denudant work with effect. 

 Moreover, we are forced to believe that the faults and 

 breaks, due to the shrinkage of the earth's crust, have 

 materially assisted in giving the earth its present sur- 

 face conformation. Few geologists in these countries 

 now give any credit to faults, joints, and the like, as 

 having assisted in forming valleys or lake-basins, 

 while none give them the prominence to which they 

 appear entitled; but on the contrary, it has been 

 stated, " there is no necessary connection between 

 fractures and the formation of valleys." It has also 

 been stated, that in many cases the materials "washed 

 out of the different valleys could not have been origin- 

 ally softer than the materials of the intervening ridges, 

 as the corresponding strata on the opposite sides of the 

 valleys are of equal hardness." The latter statement, 

 however, quite ignores the fact, that if portions of the 

 ground are broken up and loosened in lines, by systems 

 of faults or joints, the intervening parts may be still 

 firm and unbroken; and when the shattered parts 

 are denuded away, the others would remain standing 

 in unbroken ridges and masses. 



If meteoric abrasion is the universal denudant 



