393 



Notes on the Formation of extensive Conglomerate and 

 Gravel Deposits. By Henry T. de la Beche, Esq. 

 F.R.S. &c. 



( Extracted from the Philosophical Magazine of March, 1830 .) 



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

 logists considered adequate to the production of nearly all the 

 phaenomena which we observe in the structure 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 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 on 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. Playfair 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 there can be no motion of the 

 water sufficiently rapid to produce the attrition required to 

 give a round figure and smooth surface to hard and irregular 

 pieces of stone."* Although it is acknowledged that no tri- 

 turation of rock fragments into rounded gravel is now effected 

 in the bottom of the ocean, it has been supposed that gravels 

 formed or collected on the shores of continents or islands are 

 conveyed there, to be consolidated, and converted into beds of 

 conglomerate. An attention to the effects of seas on their 

 coasts will, however, show us that these gravels do not travel 

 outwards into great depths, but that the ocean exerts its power 

 to throw them back upon the dry land whence they were de- 

 rived. Attention to a sea coast with a shingle beach during 



■ Illustrations of the Huttonian Theory, p. 7. 



3 D 



