GORGES AND RAVINES. 163 



enormous length of time for their formation ; while 

 in one instance, that of Lough Ashangi and its 

 tributary streams, the author cannot account for the 

 disappearance of the detritus excavated out of the 

 tributary valleys. If, however, it is allowed that 

 rocks, when brought up to the surface of the earth, 

 contract more or less, and form open fissures not 

 necessarily vertical displacements, but only breaks in 

 the continuity of the beds such gorges as those in 

 Abyssinia would be easily accounted for, without 

 either a long period of time being expended in their 

 formation, or any large quantity of matter having been 

 denuded out of them. 



In such a narrow valley as the fissure formed by 

 the contraction of a mass of rocks, denudation from 

 water would principally take place at the bottom, 

 deepening it more quickly than it would widen it, 

 except in a very damp climate, where a protecting 

 envelope rapidly grows which might prevent the 

 bottom of the ravine from being denuded ; this may 

 be due to different circumstances. If trees or other 

 protecting plants grew on the tops of the cliffs, all 

 meteoric abrasion would be prevented from working 

 downwards ; consequently it could only act horizon- 

 tally, or inward from the sides of the ravine. Or a 

 hard rock may cap soft ones soft sandstones in 

 Abyssinia being under bedded igneous rocks this 

 also would prevent denudation from above. Besides, 



