156 Geological Society : — 



sited, their chemical composition, and the mineral condition of the 

 fossils, having been described in detail, Dr. Duncan proceeded to 

 discuss the explanations that could be suggested to account for the 

 formation and subsequent disappearance of the crystals. He came 

 to the conclusion that the mineral had resulted from the action of 

 sulphuric acid, contained in percolating water, on preexisting car- 

 bonate of lime, the sulphuric acid having been formed by the oxida- 

 tion of sulphuretted hydrogen by the oxygen evolved from the de- 

 composing vegetable remains occurring in the Plant-beds interca- 

 lated in the strata containing Selenite-spaces. The hydrocarbons 

 resulting from the same decomposition would in solution be suffi- 

 cient to produce the decomposition of the Selenite. In conclusion 

 Dr. Duncan urged that, if his explanations were accepted, the oc- 

 currence of Selenite in a deposit must be held to prove the former 

 existence of organisms in it, and the removal of the Selenite to be 

 equivalent to the loss of the evidence of such existence ; therefore 

 there can be no reason why the purest clay-slate may not have been 

 once as fossiliferous as the Woolwich Beds. 



2. "On the Relation of the Chillesford Beds to the Norwich 

 Crag." By the Rev. O. Fisher, M.A., F.G.S. 



The geological position of the Chillesford Clay has never been 

 definitely settled. Mr. Prestwich, who first described it, left the 

 question open as to its identity with the Norwich Crag, or with the 

 more recent marine, freshwater, and land series which immediately 

 underlies the great northern Clay-drift of Norfolk. Sir C. Lyell 

 supports the former view, while Mr. S. V. Wood, Jun., considers the 

 Chillesford Clay a local member of his " Middle Drift." The 

 author described the Chillesford Beds as they occur at Chillesford, 

 and thence traced them northward to Aldborough. At Thorp, north 

 of Aldborough, the Norwich Crag is exposed ; and the main object of 

 the paper was to show that this bed probably overlies the Chillesford 

 Clay. In order to prove that this Crag is not identical with the 

 Mya-bed below the clay, Mr. Fisher cited its greater thickness, its 

 difference in lithological character, and the dissimilarity of their 

 fossils ; he also remarked that it rested upon a loamy clay, and con- 

 tained a strong spring at its base, whereas the Mya-bed was always 

 observed to rest on porous beds ; he therefore inferred that this 

 loamy clay was the Chillesford Clay, and showed that the gentle dip 

 to the north would bring it into the required position ; moreover 

 he had found indurated nodules of loam, resembling weathered Chil- 

 lesford loam, in the base of the Norwich Crag at this locality. 

 Mr. Fisher next noticed the occurrence of the same beds at South- 

 wold, and stated that the well-known deposit from which the late 

 Colonel Alexander obtained so many mammalian remains was the 

 Mya-bed. The Norwich Crag is also seen in this neighbourhood at 

 Wangford, differing in character from the Mya-bed, and resting on 

 a loamy clay resembling, and probably identical with, the Chilles- 

 ford Clay. The sequence of these beds is therefore, in descending 

 order:— (1) Norwich Crag; (2) Chillesford Clay ; (3) Mya-bed ; 

 (4) Red Crag. 



