1862.] JUKES — KIVER-VALIEYS. 891 



course, and across ridges of hills that elsewhere rise unbroken far 

 ahove the level of the valleys ? This question I now proceed to en- 

 deavour to answer. 



A. The Bavines not caused hy Disturbance. — These ravines are not 

 fractures caused by internal disturbance. Transverse fractures do 

 occur in some places in the neighbourhood of some of the ravines, 

 forming N. and S. faults, and they may possibly have had some in- 

 direct influence in determining the position of the ravines in their 

 neighbourhood. Faults, however, never produce open gaping fissures 

 two or three hundred yards in width, and could only operate towards 

 the production of such ravines by inducing the erosive agencies to 

 act along their line rather than elsewhere. 



The ravines were not caused by fissures that opened at the surface 

 without producing any dislocation ; for such fissures must end gra- 

 dually below and extend to an indefinite depth. There is no ap- 

 pearance of fracture in the rocks, which strike directly across the 

 ravines from side to side, and appear to be quite unbroken in the bed 

 of the river. The ravines are evidently mere squarish gaps, worn 

 down or eroded across the edges of the highly inclined beds to a cer- 

 tain depth, and there terminating abruptly, their depth being in 

 reality slight compared to their width from side to side and their 

 length from end to end. 



The ravines, moreover, are often tortuous, precisely like the bed of 

 a river worn by its own action into the rock below. 



There is also no appearance of any crack or fissure in the low 

 lands between one ravine and another along the coui^e of the same 

 river ; but if we look to a deep-seated fracture as the origin of these 

 ravines, that fracture should be apparent all along ; for the undula- 

 tions of the surface must be so slight, compared with the depth of its 

 origin, that we may feel sure they could not make any difference in 

 the different parts of its course. 



B. Melations hetiueen the actions of Denudation and Disturhance in 

 the production of the Form of the Surface of the Ground. — I think we 

 are entitled to assume the truth of the following propositions as 

 regards the mutual action of disturbance and denudation, with respect 

 both to this district in particular and the surface of all other lands. 



1. Denudation is of two kinds, marine and atmospheric. 



2. Marine denudation is effective only about the sea-level and 

 along the margin of the land. It acts with a broad horizontal move- 

 ment, tending to plane down the land to its own level. If the land 

 be long stationary, it produces long vertical cliffs about its margin ; 

 if the land rise slowly and equably, it forms gentle slopes upon it. 



3. Marine denudation cannot produce ra\ines or narrow winding 

 valleys, except as gaps or passes upon the crests of ranges of hills 

 when the neighbouring summits were islands and the present gaps 

 or passes were " sounds " or " straits " between them, traversed by 

 strong tides and currents, and a narrow arm of the sea was thus 

 made to assume a river-Hke action. 



4. Atmospheric denudation acts vertically, either by the weather- 

 ing and disintegration of rock over the whole surface of land; or by 



