INTRODUCTION. 489 



THE C^ORALLTNE CRAG. 

 Gedgravian, Boytonian. 



In his well-known paper ^ Pi'estwicli divided the Coralline Crag into eight 

 zones, characterised by distinctive groups of fossils (for which division, however, 

 both S. V. Wood, jun.j and I considered he offered no satisfactory evidence), 

 accompanied by great physiographical changes, as, for example, at one stage a 

 submergence of the Crag area with a subseqiient re-emergence amounting to 

 from 500 or 1000 feet, a view which seemed to us highly improbable.- The 

 apparently well-marked separation, moreover, between the incoherent glauconitic 

 and fossiliferous sands of the lower part of the formation and the indurated 

 ferruginous rock-bed of the upper part, has been now found to be more apparent 

 than real, the rock-bed being merely an altered condition of the shelly sands 

 caused by an infiltration of acidulated water by which the aragonite fossils were 

 removed, though their casts may be observed in places, while the calcite Polyzoa 

 and a few calcite Mollusca remained.^ 



When we examine the matrix of the Coralline Crag from different localities or 

 different levels, whether microscopically or chemically, we find no essential 

 difference in it. It is mainly of organic origin, consisting principally of the 

 comminuted shells of marine organisms or of calcareous matter derived from their 

 decomposition, with layers of perfect shells in places, nowhere presenting the 

 features of a typical deep-sea formation. 



With the exception of a thin basement bed, not everywhere present, it seems 

 to me probable that the greater part of the Coralline Crag was deposited under 

 conditions more or less uniform, in water sufficiently shallow to be within the reach 

 of currents,^ at no great distance from the western margin of the Crag sea and 

 in banks which were possibly more or less parallel with it. 



There is no evidence to show that masses of dead shells are now being laid 

 down simultaneously in British seas over large and continuous areas. They 

 originate locally, however, in comparatively shallow- water deposits, often false 

 bedded, containing stratified beds of mohuscan debris, for the most part those of 

 dead animals, accumulated under the influence of currents. 



As to Prestwich's suggestion that there may have been a submergence of from 



^ Quart. Journ. Geol. Soc, vol. xxvii, p. 171, 1871. 



^ These views were discussed by us in the lutroductiou to the 1st Suppl. to Wood's Monograph 

 (Palaeout. Soc, 1872), and by myself in the Quart. Jouru. Geol. Soc , vol. lix, p. 320, 1898. 



3 See P. F. Kendall, Geol. Mag. [2], vol. x, p. 497, 1888. 



■* Reef-building polyzoa are common at some localities in the Coralline Crag. Such forms are said 

 to flourish best in clear water agitated by currents. The late CI. Reid remarks, moreover, that the 

 whole of the Coralline Crag is more or less current-Ledded {op. cit., p. 36). 



