Vol. 54.] AND THE COEALLrNTE CRAG. 321 



nor do I think that it is possible to do so ; he stated, however, but 

 as a matter of opinion only, the horizons to which he considered 

 the beds occurring at other localities should be referred. He 

 claimed that these supposed zones are characterized by distinctive 

 groups of fossils, and that their deposition was attended by 

 great physiographical changes, including a submergence of the Crag 

 basin, by which at one stage the North Sea attained a depth of 

 from 500 to 1000 feet, while at another the climate of Northern 

 Europe was refrigerated sufficiently to permit of floating ice 

 reaching East Anglia, with boulders from either Scandinavia or 

 the Ardennes. 



The Coralline Crag, both in the Sutton outlier and in the main 

 mass of the formation, seems to fall naturally into two divisions (a 

 view held by all observers up to the present time), the lower consisting 

 of shelly incoherent sands, generally of a whitish colour, and the 

 upper of a porous ferruginous limestone, soft and friable when first 

 quarried, but acquiring a hard crust by exposure to the air.^ The 

 colour of the latter, normally a light ochreous yellow, assumes on 

 further oxidation a dark rusty hue. Specimens of Pecten and 

 other mollusca whose shells are composed of carbonate of lime in 

 the form of calcite (the translucent variety), and the remains 

 of polyzoa, are common in this rock-bed, as they are also in the 

 shelly sands, but the opaque or arragonite mollusca are represented 

 in it by casts only.^ These casts occur, however, in many places 

 and at different levels, generally in layers, and often in great 

 abundance. 



At first sight nothing could well seem more distinct than the 

 soft shelly beds of the lower, and the hard ferruginous rock of the 

 upper part of the Crag, but I now believe that the difference is 

 more apparent than real, and that the rock-bed is merely an altered 

 condition of the shelly sands,^ its ferruginous character being due 

 to the infiltration of water charged with oxide of iron, arising from 

 the decomposition of part of the glauconite of the unaltered Crag. 

 Indeed, I have lately discovered, in an important section at Iken, 

 which will be described later on (see pp. 338, 339), the two kinds 

 of Crag side by side, and passing into each other. 



Prestwich separated from the rest the upper or ferruginous 

 portion of the Crag, which, as will be seen by the sections (figs. 5 

 & 7, pp. 328 & 332), forms nearly one half of the entire mass of 

 the formation, calling it zones G & H. The nodule-bed at the base 

 of the Crag, found in one spot only at Sutton, he described as 



^ It sometimes becomes sufficiently hard to be used for building, as for 

 example in the tower of Chillesford Church. 



2 See P. F. Kendall, aeol. Mag. 1883, p. 497. 



^ More than 20 years ago attention was simultaneously called by Mr, 

 Whitaker, and by Mr. Wood & myself (Quart. Journ. Geol. Soc. vol. xxxiii, 

 1877, pp. 75 & 122), to the alteration of the Red Crag by infiltration ; and 

 M. Van den Broeck, about the same time, made similar observations in Belgium. 

 It is not a little strange that it occurred to none of us that the Coralline Crag 

 might have been affected in the same way. 



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