616 ME. C. B. TVEDD ON THE [jS'oV. 1 898, 



reason against its passing under and forming the base of the 

 Corallian Oolite, except perhaps the thickness of oolite under which 

 it would have to pass. It is worth noting, too, that although there 

 were signs of the proximity of clay in the blue oolite from the 

 bottom of the well at High Pen Farm, no trace of ferruginous oolitic 

 grains was found, and no diflPerence of fauna from that of the Northern 

 Quarry except the greater abundance of fossils and the presence 

 of one coral-specimen which, as embedded in its matrix, might be 

 either a Montlivaltia or a Thecosmilia^ probably the former. 



In favour of the view that the Elsworth Eock passes laterally into 

 the Corallian Oolite there are these considerations : — The Elsworth 

 Bock, though found at several places to the west, is not known 

 directly north or east. It is known to be absent at March, where 

 a deep boring ^ showed continuous clay from the Kimeridge into the 

 Oxford. Prof. Sedgwick stated that several wells were sunk 

 between Cambridge and King's Lynn through Kimeridge and Oxford 

 strata without meeting any Corallian. The Elsworth Pock, then, is 

 absent at no great distance north and north-east of its exposure at 

 the southern end of the Corallian ridge of Upware, an area of great 

 change from the prevalent clay-conditions of this part of England, 

 and about the horizon of the Elsworth Pock. It is therefore here, 

 if anywhere, that we might expect the Elsworth Rock to alter in 

 character. Moreover, it seems more natural to account for the 

 introduction of this thick Upware oolite in the midst of Jurassic 

 clays, as an expansion of the Elsworth Pock, than to assume the 

 presence of both at the same spot and leave the former unaccounted 

 for. The continuous Sei^inda-\iedi at the top of the Oxford Clay and 

 the occurrence of lumps of clay in the lower part of the Oolite at 

 High Een Earm suggest the possibilit}^ of a pause in deposition such 

 as that indicated by the nodule-bed at the base of the Kimeridge, or 

 of an eroded basin, as in the Gault below the Cambridge Greensand. 

 If the deposit which has formed the Elsworth P,ock passed into 

 some such local basin, it might well change in character at once 

 rather than continue at first of the same type. Still, I am well 

 aware that no proof has been given here of a lateral passage, and I 

 am content to wait for further evidence which may be forthcoming 

 at Upware. 



The Upware Corallian, then, seems to represent the whole of the 

 Ampthill Clay of Bedfordshire and Lincolnshire. In the district 

 between Elsworth, St. Ives, and Upware, and perhaps farther south 

 and east, the lower part of the Ampthill Clay develops into a 

 limestone of Elsworth type which is closely associated with the 

 Upware Corallian and may pass into the Corallian Oolite. In any 

 case, considering the greater thickness of the Yorkshire Corallian, it 

 would seem to belong to a higher horizon than the Lower Calcareous 

 Grit. 



In conclusion, I wish to express my thanks to Messrs. H. 

 H. Thomas and G. McFarlane, of Sidney Sussex College, for 

 1 Mem. Geol. Surv. S.W. Norfolk & N. Oambs. 1893, p. 154. 



