Correspondence. 281 



ancient occupation of the land by the sea. The same fact furnishes 

 a presumption, in the absence of very strong evidence to the con- 

 trary, that those hollows and portions of valleys in Wales and the 

 Lake district, which have been scooped out in rocks as hard, if not 

 harder than the basalt of Auvergne, have not been excavated by any 

 process of atmospheric denudation which is not admitted to involve 

 such an enormous lapse of time as to allow the sea to outstrip it in 

 its march, and take the work out of its hands. The sea has, com- 

 paratively, little difficulty with hard rocks. It insinuates itself into 

 joints and crevices, undermines, detaches, and carries away the 

 largest blocks in a wholesale fashion ; whilst atmospheric agents, 

 unless very powerfully assisted by gravitation, have to grind down, 

 or break up, before ihej can effect that transportation which consti- 

 tutes the main part of denudation. I have only farther, in reply to 

 Mr. G. Poulett Scrope, to state that I have not denied the power of 

 rivers to form cliffs, but asserted that there are many inland cliffs 

 which the sea only could have formed. 



The letter of Mr. J. B. Jukes displays the modesty of a true 

 philosopher, animated solely by a desire to arrive at truth. Though, 

 apparently, widely differing, I think we are more or less agreed on. 

 the following great fundamental points : — T'irst, that in certain areas 

 the sea forms plains and table-lands ; that there cannot be plains with- 

 out surrounding escarpments or acclivities, or table-lands without decli- 

 vities or cliffs, in both cases more or less indented, and here and there 

 shaped into bays and combes. Second, that in other areas the sea 

 produces smoothly- swelling elevations and depressions, now and then 

 varied by projecting rocks, or rocky knolls. Third, tliat if certain lands 

 continue a long time above the sea, their coasts must be long exposed 

 to the action of waves, tides, and currents, and that in this way great 

 inequalities must be produced. Fourth, that in partially-submerged 

 areas, soiuids or straits raust be excavated by tides and currents ; and 

 that these, on their being elevated, must become momitain gaps or 

 passes (see Mr. Jukes's own admission in Quart. Journ. Greol. Soc, vol. 

 xviii., page 391). I cannot therefore understand why Mr. Jukes 

 should not admit that many escai-pments and upland rocky cliffs 

 (which are not the immediate sides of river- valleys) ; all our smooth 

 curvilinear combes (which rain-torrents only tend to furrow, dis- 

 figure, and destroy) ; and many of our momitain passes, are not the 

 result of marine denudation. Since I commenced making systematic 

 observations in Central Wales, I have gradually been convinced of 

 the necessity for allowing that ravines on the sides of table-lands 

 have been mainly excavated by streams but these ra^T.nes are gener- 

 ally of the V form. A continuation of the process would, un- 

 doubtedly, wear down very deep vallej^s ; but that wide and flat-bot- 

 tomed valleys, and connecting gorges or passes, have been excavated 

 by rivers, ajopears to be contradicted by the fact that, in those I have 

 examined, there is an absence of true river shingle at any great 

 height above the present river level. In many parts of the upper 

 valley of the Wye, especially between Builth and Ehayader, I have 

 found river-sliingle running up to heights, varying from a few feet 



