Vol. 67.] DURING THE CRETACEOUS AND EOCENE PERIODS. 1^7 



large casts of the univalve Gisortia, and of species of the small 

 sea-urchin Sismondia. 



The Gisortia Bed is of considerable significance, both on account 

 of its topographical importance and of its wide distribution. In 

 my own expedition I obtained that large- sized member of the 

 Conidae (Gisortia) on the eastern side of the great gravel desert to 

 the west of the Moela Oasis ; and also on the western side of this 

 expanse, in the Bahr district close to the northern edge of Baharia 

 Oasis. Mr. Beadnell records it west of Fashn as a silicificd 

 limestone with large silicified Conidae, 1 and Dr. Ball gives prac- 

 tically the same description of it, describing it as forming the 

 plateau-rock near the great belt of sand-dunes. 



In Moela it had already aroused the attention of Mayer-Eymar ; 

 but Dr. Blanckenhorn makes no allusion to any stratum of this 

 nature in the area studied by him in detail on the eastern bank of 

 the Nile between Helwan and Minia, nor has it been noted in the 

 Moqattam section. It re-appears, however, in the desert between 

 Cairo and Suez : for, not only is it mentioned in Barron's Geological 

 Survey Memoir describing this region (1907), but under the same 

 name as that adopted by me before I was aware that the term 

 had been previously used by him. 



Near the top of the cliff, at the well-known Gebel Genefe, near 

 Ismailia, is a bed containing Sismondia scemanni Desh. and Gisortia 

 gigantea Munst., these being overlain by a coral-layer. In the 

 plateau bordering Gebel Ataqa the upper beds again contained many 

 Lucince, under these being the Dendracis-conferta Bed yielding 

 Sismondia scemanni and other echinid spines. It, in turn, overlies 

 the Gisortia- gig antea Bed (Barron, op. cit. pp. 93, 95). In Blancken- 

 horn's work the nearest resemblance to this stratum is one recorded 

 as the summit-rock of the Lower Moqattam Series (I. 5) : where a 

 yellow sandy limestone (with Sismondia cf. logotlieti, Thagastea 

 luciani de Lor., and a number of other species) overlies a hard white 

 limestone with corals, as in the summit-bed of the Northern Galala 

 and Gebel Awebed. 2 This Dictyoconus Bed, as he terms it, appears 

 to have close relations to the Gisortia stratum. 



Enough has been said to show that this white limestone is of 

 prime importance ; and, wherever recognized, this reef-formation 

 practically marks the close of the Lower Moqattam Series. 



The beds which underlie this stratum in the desert south-west of 

 the Fayum are very variable in character. The first section exa- 

 mined in the cliff south-west of Beni Suef consisted of : — 



(Top.) 1. The snow-white Gisortia Bed, at least 2 metres thick, containing 

 small nnmmulites, Eschara duvali Mich., and broken Purocidaris 

 spines. 

 2. Yellow-tinted cavernous limestone rich in small nummulites andGryphaa 

 pharaonum Oppenh., with, in places, large bent Carolia. 



1 J. Ball & H. J. L. Beadnell, ' Baharia Oasis ' Egypt. Geol. Surv. Mem. 

 1903, p. 22. 



2 See M. Blanckenhorn, ' Neues zur Geologie & Palaontologie .Egyptens ' 

 Zeitschr. Dentsch. Geol. Gesellsch. vol. lii (1900) p. 445. 



