Mr. De la Beclie on the Excavation of Valleys.. 243 



mouth, which forms the sole channel of drainage to a district 

 many miles in length. The actual force of this stream, even 

 with every assistance from floods and rains, has not accom- 

 plished more than a cut varying from four to fifteen feet deep, 

 bounded by perpendicular walls : these walls composed for the 

 most part, not of the lias strata that have been widely exca- 

 vated, but of flint and chert gravel, and drifted materials such 

 as are strewed over the valley at all heights, from the bed of 

 the actual river to the tops of the hills. The question may be 

 asked, why, if some solvent power has been able to produce 

 the unrolled gravel on the summits of the hills, it has not been 

 able to cause the valleys themselves. If these valleys in the lias 

 had been equally covered by a hreche en place, composed of 

 fragments of lias, it might be urged that they also were pro- 

 duced by dissolution of the lias. No such breccia has been 

 found in them ; and the only remaining adequate agent seems 

 to be a voluminous mass of moving waters, to the duration of 

 which 1 will not venture to assign a time. This seems to have 

 acted on the rocks in proportion to their hardness and com- 

 position. 



Such valleys as those of Lyme and Charmouth occur in all 

 countries where nearly horizontal strata have not been much 

 disturbed ; and the causes that produced them seem to be the 

 same with those that have also operated extensively upon the 

 great escarpments of strata, leaving outliers and other marks 

 of former continuity, which some great overwhelming force 

 lias interrupted*. 



II. — Valleys of Excavation in Jamaica 'which cannot he referred 

 to Rains or Rivers. 

 Depressions on the earth's surface existing when the present 

 order of things commenced, would become channels of drain- 

 age to rain-water accumulating into streams and rivers. There 

 are however depressions in which not even a rivulet at present 

 Hows, and of these we have examples in the white limestone 

 districts of Jamaica, where the inhabitants are compelled to 

 obtain water exclusively by collecting the rain in tanks ; yet 

 in these districts the natural inequalities of the land present 

 the same ibrms of hill and dale as occur elsewhere ; and even 

 the violent rains in this tropical climate form no continuous 

 rivers, but are swallowed by numerous sink-holes or natural 



• This force seems to liave liccn exerted very generally; for in all conn-. 

 inn there arc ine<|Uiilitics of siirfaL'c, iii(lc|)cmlcnt of slralification : and it 

 is by no means uiicoinmon to see the liit;lier parts of curved and contorted 

 strata I'cmovcd, so thai in sections strictly rcpresentinf; tlicin, we are oblij^cd 

 to add inia;;inaiy duited lines to render the curvatures intellii^ible to pcr- 

 fons unaccu^luiiied to geological investigations. 



'J 1 2 cavities 



