Vol. 54.] LBNHAM BEDS AND CORALLINE CKAG. 327 



North Sea area, and finally died out altogether, their place being 

 taken in the newer beds by arctic and boreal forms, unknown from 

 the older formation. 



The principal difference between the various portions of the lower 

 -or unaltered condition of the Coralline Crag seems to be that in some 

 the mollusca are chiefly of large, in others of smaller species, a third 

 variety of Crag being composed of comminuted material only. Certain 

 localities, however, as for example the Gomer pit, are characterized 

 by an abundance of univalves, which generally are much less 

 common than bivalves in these beds, but such differences are by 

 no means persistent. Layers of large shells, containing especially 

 ■Cyprina islandica, occur in all parts and at all levels in the Crag, 

 as will be seen hereafter, and I am unable to find any bed which is 

 continuous except for a short distance, nor is this to be wondered at. 

 It is not probable that at any one period the sea-bottom was for 

 some miles continuously covered with a thin layer of the dead and 

 drifted valves of Cyprina and other large molluscs, and that then 

 these species disappeared for a time from the Crag area. Shells 

 ;are sorted out by currents of varying strength, just as pebbles 

 are in beds of gravel, small specimens naturally accumulating in 

 one place, larger ones in another, and comminuted shells or fine 

 -calcareous sand in a third.^ 



There are localities in the Crag, however, at which it seems at 

 first sight that beds, the fossils of which are more or less of a 

 similar character, may be traced continuously from one section to 

 another, as, for example, between the Broom pit at Gedgrave 

 (No. 11 of map, fig. 4) and the Hall pit (No. 12) in Sudbourne Park. 

 At each of these we find one seam with Cyprina and other large 

 molluscs, and another seam immediately over it containing princi- 

 pally smaller shells, both being on about the same level at the two 

 places. To an observer who believes, as Prestwich did, that the 

 drag rests on the more or less horizontal surface of the London 

 Clay,^ these would seem to be the same, but between the two pits 

 the base of the Crag dips 8 or 9 feet (see fig. 5, p. 328), and thus 

 beds which appear to be in correspondence are not really so. 



I have attempted, from the notes given in Prestwich's memoir, 

 to construct a diagram (fig. 6, p. 329) showing the structure of the 

 Coralline Crag from E-amsholt to Aldeburgh according to his views, 

 but the result does not seem to me to lend much support to the zone- 

 theory. 



In the table on pp. 330-1 are enumerated the features upon which 

 Prestwich relied to justify the separation of the Coralline Crag into 



^ In the Natural History Museum at South Kensington there is a large block 

 of limestone from the Calcaire grossier, similar in character to the shelly sands 

 of the Coralline Crag, containing in profusion the drifted and stratiiied shells 

 of mollusca, which are more or less of the same size throughout, and but few 

 univalves. Both univalves and larger bivalves were present in the seas of that 

 period, as may be seen by reference to one of the wall- cases close by. None, 

 however, were deposited at the spot from which this block was taken, the 

 selective power of the currents which there prevailed having sorted out only 

 forms of a certain size and weight. 



2 See his section from Sutton to Iken, Quart. Journ. Geol. Soc. vol. xxyii 

 (1871) pi. XX. 



