MOVEMENTS OF ROCKS, ETC. 793 



substantially the same level as that of the surrounding plain, the 

 mountain originally formed having quite disappeared *. 



The source of additional jointing and fissuring which would 

 account for the greater rate at which these uplifted rocks have been 

 disintegrated is evidently to be found in the lateral strain and 

 stretching to which over a long period they were subjected. Some 

 increased weakening will also have been caused by the presence of 

 precipices and steep slopes in the way before explained. 



If the production of fissures by horizontal movement, unattended 

 by upheaval, has been as common an occurrence as the foregoing 

 would lead us to conclude, it may, I think, be fairly questioned whether 

 sufficient prominence has of late been given to the influence exerted 

 by it in determining the directions taken by rivers and streams. 



The opening of a very narrow fissure across the bed of a river 

 might suffice to initiate the complete diversion of the course of the 

 water, and, in cases where no faulting or upheaval accompanied the 

 fissuring, there would commonly be no evidence to betray the origin 

 of the diversion. 



Unfilled fissures have often been produced concurrently with 

 dykes in modern times, and must have been frequently produced in 

 the past. And we have in some cases a correlation of the locality 

 and direction of dykes and the direction of watercoursesf pointing to 

 a common or connected origin. 



Moreover, I have suggested that the existence of elevated ground 

 over plastic rock causes the spread of the latter to be more con- 

 siderable on account of the greater weight pressing upon it, so that 

 fissuring by this means will commonly have been more prevalent 

 among mountains and elevated lands than elsewhere. And this 

 would furnish an explanation of the well-known fact that gorges, 

 ravines, and canadas are found in every high country, and also go far 

 to account for the great number of cases of rivers intersecting 

 elevated and isolated rocks. 



In harmony with this explanation, we find that the examples of 

 rivers whose courses are thus out of conformity with the features of 

 the land- surface and also with the dip of the strata are most 

 numerous in countries where dykes and other traces of the presence 

 in the past of very plastic or fluid rock near the surface are found. 



Thus, in the country of the great canons in IS'orth America we 

 have innumerable instances of want of conformity between the 

 courses of considerable streams and the contour of the ground- 

 surface and the dip of the strataj, and in the same district we have 

 a recurrence over wide areas of similar and evidently related 

 phenomena of faulting and contortion, which indisputably proves 



* * Geology of the Henry Mountains,' pp. 25, 33, & 35. 



t In Scotland, for instance. 



X ' Tertiary Hist, of the Grand Canon District' (0. E. Button), pp. 2, 49, 60, 

 73,201, 203, 204, 220 ; and ' Geology of the High Plateaus of Utah,' pp. 17, 257. 

 Keport of the U.S. Geological and Geographical Survey of the Territories, 

 Colorado, &c., 1876, pp. 52, 54 ; ' Geology of the Black Hills of Dakota,' p. 216. 



