THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY. 



[NEW SERIES.] 



MARCH 1830. 



XXIII. Notes on the Formation of extensive Conglomerate and 

 Gravel Deposits. By Henry T. De la Beche, JFl/i.^'. Sfc* 



[With a Plate.] 



A T the present time, when actual causes are by some geo- 

 -^^ legists considered adequate to the production of nearly all 

 the phaenomena which we observe in the structui'e of the earth's 

 crust, it becomes important to ascertain, as far as our know- 

 ledge will permit, the value of such causes, and thence judging 

 how far the whole, or any part of them, may have been capa- 

 ble of forming the rocky masses of which the surface of our 

 planet is composed. 



It has been imagined that extensive conglomerate and gravel 

 deposits are owing to causes similar to those now existing; in 

 some cases to the joint action of rivers on their beds and seas 

 on their shores, and in others solely to the action of the former. 

 To ascertain if such causes could have produced such effects, 

 we should examine the present action of seas on their coasts 

 and rivers on their beds, so far as regards the production of 

 rounded gravels. 



I. Action of Tidal Seas on their Coasts. 

 It has long been known, and often remarked, that seas gain 

 on some coasts and lose on others ; in other words, that seas 

 cut away and destroy rocks, even the hardest, in some places, 

 and pile up the detritus, acquired either from this destruction, 

 or from rivers, in others. PJayfair has well observed, that 

 rounded gravel " can only be found in the beds of rivers, or 

 on the shores of the sea ; for in the depths of the ocean, though 

 currents are known to exist, yet tliere can be no motion of the 



• Communicated by the Author. 

 M.S. Vol. 7. No. 39. Marc/i 1830. Y water 



