248 Mr. De la Beche o?j the Excavation of Fallens. 



now in the act of cutting a gorge, which, if time be allowed, 

 may let out the waters of the lake above it. If this should ever 

 be accomplished, the gorge will resemble those we have been 

 describing, and show equally with them, that existing rivers 

 may excavate gorges and precipitous channels, but that these 

 excavations are entirely disthict from valleys of denudation. 

 In all such cases as this, and in the minor effects of meteoric 

 influence, we have gorges, ravines and gulleys, cliffs, taluses 

 and landslips, — all tending to destroy the more or less rounded 

 forms of anterior valleys which were excavated by a foi'ce act- 

 ing generally and with enormous power ; a force scarcely re- 

 ferable to any other cause than a voluminous mass of over- 

 whelming waters. 



P.S. I admit that considerable changes have been and con- 

 tinue to be effected on the earth's surface by causes actually 

 existing. In the time of hurricanes, tropical rains effect that 

 which an inhabitant of milder regions would scarcely credit. 

 In Jamaica, the great hurricane of 1815 produced numerous 

 cliffs and landslips in the mountains of St. Andrew and Port 

 Royal. The gulleys also in this island are very numerous and 

 deep, particulai'ly in the great gravel plains. This gravel the 

 torrents do not produce, but only tend to cut up and destroy ; 

 so also do the rivers which traverse it; the effect both of rivei's 

 and torrents being to make precipitous excavations not only 

 in stratified rocks, but also in these beds of gravel, the origin 

 of which must be referred to some more powerful, more ge- 

 neral, and more ancient cause. 



Although I consider that many gorges have been cut by the 

 gradual discharge of lakes, and by the rivers that now flow in 

 them, I by no means suppose that all gorges or ravines have 

 been thus formed : many evidently were not ; and of these, some 

 have rivers now flowing through them, others contain no stream 

 whatever. The gorge of Clifton near Bristol, through which 

 the Avon passes, may be cited as an example of the first kind ; 

 if this were closed, the resulting lake would be drained in the 

 dii'ection of Nailsea, and exert no action on the rocks of Clif- 

 ton. The carboniferous limestone districts of England abound 

 in examples of the second kind; viz. of gorges entirely dry, or 

 through which the rills now passing are much too insignifi- 

 cant to have caused them. 



XXXVI. On 



