THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY. 



[NEW SERIES.] 



OCTOBER 1829. 



T 



XXXV. Notice on the Excavation of Valleys. By Henry T. 

 De la Beche, F.U.8. ^'c* 



[With a Plate.] 

 WO opinions have been entertained by geologists, as to 

 the causes that have excavated valleys : some contending 

 that they have been produced by the rivers that now run in 

 them, aided by the bursting of lakes and meteoric agents; while 

 others consider that the greater proportion of such valleys 

 have been formed by what has been called diluvial action, and 

 by other causes operating at the bottom of ancient seas. It 

 appears to me that these two rival theories may be reconciled 

 with the facts presented by nature, and that both are, to a 

 certain extent, correct. It would, I think, be almost impos- 

 sible to deny that rivers, more particularly those discharged 

 from the many lakes that probably once existed, have cut 

 deeply into the land, and have formed gulleys, ravines, and 

 gorges : but again, it seems utterly at variance with the rela- 

 tions of cause and effect, to suppose that valleys, properly so 

 called, could have been formed either by the discharge of la- 

 custrine waters, or by the rivers that now run, or could ever 

 have run, in them. 



In the discussion of this subject, we should consider only 

 such valleys as, by the correspondence of horizontal or nearly 

 horizontal strata on their opposite sides, show that the strata 

 were once continuous, and that their continuity has been de- 

 stroyed by the removal of the intermediate portions; — of course, 

 the very numerous valleys formed by rents and contortions, 

 and such as iiave been termed vdleys of elevation and depres- 

 sion, us well as those of original formation, do not enter into 

 our present consideration. 



• C^ommimicatod by the Aullior. 

 .V..V. Vol. G. No. ,'5i. Oct. 1829, 2 I It 



