166 F. P. Mennell— 



borings proved the Upper Gault to rest on the basal bed of the 

 Sandringham Sands. This points to considerable erosion of the 

 Lower Greensand prior to the deposition of the transgressive clays. 

 At West Dereham and Mouse Hall (" Muzzle "), only a few miles 

 to the north, a thin representative of the mterruptus-zone, not more 

 than 3 feet thick at Mouse Hall, rests directly upon the sandy 

 mammillatus-hed. At both localities we have obtained the typical 

 fossils of these two horizons. At Mouse Hall this remnant of the 

 interruptus-zone is overlain by very pale Upper Gault Clay, which 

 was at one time mistaken for Chalk Marl. This clay yields 

 Inoceramus sulcatus Park, and other fossils of the Upper Gault in 

 its lowest part. It is evident that the base of the overlapping Upper 

 Gault here rests upon a thin remnant of the Lower Gault which has- 

 locally not been wholly removed by the erosion accompanying the 

 overlap. These appear to be the most northerly exposures of the 

 Lower Gault known in this country. 



North of West Dereham the Upper Gault thins rapidly and becomes 

 highly calcareous. Few good exposures are to be seen until the 

 neighbourhood of Grimston is reached, some 14 miles to the north. 

 Here the characteristic fauna of Bed IX of Folkestone is found a few 

 feet above the base of the overlapping strata, which rest on 

 unfossiliferoiis Lower Greensand. ^ At Dersiagham the Upper Gault 

 is reduced to 7 feet in thickness and there shows increasing hardness 

 and the introduction of the red coloration which becomes so 

 characteristic at Snettisham and Hunstanton to the north. It 

 becomes evident now that we are dealing with a thin calcareous 

 equivalent of the overlapping argillaceous Upper Gault. The remark- 

 able reduction in the thickness of the Upper Gault may be seen 

 by a comparison of a series of well-sinkings and borings between 

 Stoke Ferry and Snettisham. Within this distance there is a 

 reduction in thickness from 56 feet at Stoke Ferry to 3 ft. 6 in. at 

 Snettisham. 



{To he continued.) 



The Northward and Eastward Extension of the 

 Karroo Lavas. 



By F. P. Mennell, F.G.S., M.I.M.M. 



rpHE lavas which cap the Karroo beds of Southern Africa have 

 -*- general interest as indicating the eruptive activity prevailing 

 at a time of complete quiescence in the European area. The 

 sedimentaries immediately beneath them (Stormberg series) are of 

 Rhsetic or even Liassic age, so that the lavas themselves, which are 

 many thousands of feet thick in the Drakensberg range, may con- 

 fidently be ascribed to the Jurassic period. 



^ W. Hill & A. J. Jukes-Browne, "On the Lower Part of the Upper 

 Cretaceous Series in West Suffolk and Norfolk " : Quart. Journ. Geol. Sac, 

 vol. xliii, 1887, p. 551. 



