160 Wyatt-Edgell — Lichas. . 



and done so little in the gorge ? Many river-conrses which do not 

 embrace plains, consist of a succession of wide basins and narrow 

 connecting gorges, where the rivers ought to have formed continuous 

 valleys, supposing them to have been the excavating agents.^ All 

 these phenomena can be easily explained by supposing the plains to 

 have been arms of the sea, the wide valleys or basins inland seas 

 and bays, and the connecting narrow valleys or gorges, straits. 



Tlain. 



B 



Fig. 2. 



That rivers have not excavated large valleys of the form repre- 

 sented by Fig. 1 is, I think, evident, not only from the fact that 

 they have done so little on plains, but likewise from the similar 

 form of their actual channels in the valleys and on the plains. I 

 cannot see why the channel A, Fig. 1, should not be regarded as the 

 measure of fluviatile denudation as much as the channel B, Fig. 2. 



Eeserving for a future occasion some additional remarks on the 

 river-courses of Wales, the Lake District, and other parts of 

 England, I would conclude this article by again directing at- 

 tention to the simplicity of the marine contrasted with the pluvial 

 and iiuvial theory of denudation. The latter, regarded as embrac- 

 ing only one period of emergence, requires such a combination 

 of disintegTating and carrying agents, and such a correspondence 

 between local elevatory movememts and inclinations in river-chan- 

 nels, to keep its complicated machinery going, and prevent a glut 

 of detritus, as to render it untenable without very strong evidence 

 in its favour, consisting of an extensive collection of instances, 

 and a wide indication of facts ; and when we come to connect 

 former with recent periods of emergence, the theory becomes still 

 more entangled. The reiterated and locally-unequal movements, 

 necessary to submerge and re-elevate a given area, accompanied 

 by the increase or obliteration of pre-existing inequalities through 

 the action of the sea, must be sufficient to derange the effects of 

 any former system of subaerial denudation, so as to render a 

 resumption of its operations impossible, and a fresh beginning 

 necessary. 



rV. — On a Species of Lichas, and other new foems feom the 

 Llandilo Flags. 

 By H. "Wyatt-Edgell, .59th Eegiment. 

 T was publicly stated not long since ^ that of all the Silurian 

 strata there were no two so closely connected as the Llandilo 



I 



1 Examples of all the ahove phenomena may be found in the courses of the Ogwen, 

 Dee, Severn, Teme, Lug, Derwent (Derbyshire), "Wye, Usk, Avon (Somersetshire), 

 Exe, Dart, etc. 



2 See Professor Ramsay's Address, 1863, Quart. Journ. Geol. Soc, vol. xix. p. 38. 



