531 



He objects to the separation of the beds with shells from those with- 

 out, and shows that the shifting of a sand-bank, would correctly 

 account for the occasional occurrence of beds of sand 30 feet thick, 

 resting upon strata inclosing testacea. 



Believing that the true rationale of the crag is to be found in the 

 hypothesis of sandbanks, inhabited by testacea, and situated in a 

 tidal way exposed to violent fluctuations of the sea, as well as subject 

 to drifts of extraneous matter from land waters, the author sees nothing 

 extraordinary in the idea that accumulations of sand and shingle, may 

 have formed a part of that deposit in which the crag is regularly 

 stratified ; but he cannot consent to such accumulations, though con- 

 temporaneous with the crag, being classed under that name, much less 

 can he consent to diluvial clay being also included in it. 



If then, says Mr. Clarke, we assume that in this tertiary sea, sand- 

 banks were formed, around the shelves and under the lea of which 

 testacea collected, lived and died, as at present, many of the phe- 

 nomena of the crag may be readily solved, and we shall not need to 

 wonder why the bivalve shells are found lying with the fiat sides to 

 the strata of sand ; why the young are congregated in one group, and 

 the old in another ; why pebbles are found covered with balani ; or why 

 the remainsof terrestrial mammalia,are associated with those of whales 

 and fishes. 



With respect to Mr. Charlesworth's subdivision of the crag into 

 two ages, the author fully agrees with a qualification of the word 

 age. If that gentleman had said different periods of the same age, 

 Mr. Clarke would find no difficulty in admitting the justness of the 

 classification ; as not only species but genera of shells are differ- 

 ently grouped according to localities. The Norwich crag, he also 

 admits, differs from the Suffolk, and on the authority of Mr. Wood- 

 ward alludes to bouldered Suffolk shells occurring at Thorpe, in 

 Norfolk. 



The corals of the lower bed of the Suffolk crag, the author is of 

 opinion, betoken a warmer climate, and he says, if during the crag era 

 the earth gradually cooled, the change from a coralline deposit to one 

 more nearly related to the inhabitants of the present era, would be 

 the natural and inevitable result. 



Mr. Clarke fully assents to the observation brought forward by 

 Mr. Charlesworth in a paper, read before the British Association at 

 Bristol, on the mixed nature of the deposits now forming on the coasts 

 of England, in which the remains of the adjacent cliffs are intermingled 

 with the shells of existing species ; and adduces examples with which 

 he was acquainted, long before the reading of that paper. 



The author then offers some further remarks on the necessity of 

 separating the diluvium from the crag. He is of opinion, the gravel 

 found in the latter, if carefully observed, will be acknowledged to dif- 

 fer from regular diluvial gravel, and that its occurrence only betokens a 

 diluvial action during the period of the crag. That such actions have 

 been often repeated, he says there can be no doubt, for the beds of 

 superficial gravel of Suffolk and Dorsetshire, are evidently not of the 

 same period ; and no one who has studied gravel deposits accurately, 



