Palcpontology of the Libyan Desert, etc. 177 



will be regarded as the true normal development of the Senonian 

 division of the system. 



2. The Tertiary System. — There is no sharp demarcation line 

 between the Chalk and the Tertiary rocks in the Libyan desert. 

 There is no gap in the stratification, no interval of fresh-water 

 deposits, or any appearance of eruptive rocks to mark the interval 

 between the two systems, but the chalky sediments of the older 

 Eocene follow those of the Upper Chalk with hardly any variation 

 in their characters. And yet palasontologically the boundary 

 between the Chalk and the Eocene is clearly defined, notwithstanding 

 the continuity of marine deposits. The author had never observed 

 either in or above the oldest Nummulitic bed sa single characteristic 

 Chalk fossil; neither a Nuramulite in the Chalk strata. This 

 remarkable continuity of deposition is in striking contrast with 

 the hiatus, which, nearly everywhere in Europe, exists at this 

 horizon, and can only be paralleled with the deposits of similar 

 age in the Western territories of North America. 



A. The Eocene deposits of the Libyan desert are subdivided as 

 follows : 



I. The Libyan series (Lower Eocene). — This series is mostly of 

 a calcareous nature, though its lowest beds are occasionally clayey 

 or sandy, and then always thin-bedded and saliferous; it reaches a 

 thickness of about 500 m. Prof. Zittel separates it into a lower 

 and upper division ; the lower, typically shown in the Nile valley 

 between Esneh and Thebes, and in the border precipices of the 

 Oasis of Chargeh, has for its characteristic fossil Operculina libyca, 

 Schwager, which fills entire beds ; this species is accompanied by 

 several forms of Nummulites, Alveolina, etc. The upper division 

 of the Libyan series is best displayed in the Nile valley between 

 Siut and Cairo, and its principal fossils are long fusiform Alveolinas 

 of the group of A. oblonga ; as well as a great variety of species of 

 Miliola. This division is regarded as homotaxial with the London 

 Clay. 



IL The Mokattam series. — This name is derived from the well- 

 known Mokattara hills in the neighbourhood of Cairo, where sections 

 of the familiar limestone rocks so largely composed of Nummulites 

 are clearly shown, with a thickness of about 250 metres. The 

 distinguishing fossils are Nummulites GizeJiensis, N. curvispira, N. 

 Beaumonti and sub-Beaiimonti. The Molluscaof this series according 

 to Prof. Mayer-Eymar show that it is of corresponding age to the 

 Lower Parisian. 



In many localities in the desert the Mokattam limestones are 

 filled with flint nodules similar to those of our Upper Chalk. These 

 nodules, by the weathering away of the limestone, remain to form 

 a complete surface layer, and the Nummulites, as well as other 

 fossils, frequently occur in the same situation in a silicified con- 

 dition. Prof. Zittel noticed the remarkable fact that whilst the 

 uncovered and air-exposed portions of these fossils are silicified, 

 the covered parts remain calcareous. He says : " Limestone blocks 

 are abundant which have a crust of silica an inch in thickness, 



DECADE III. — VOL. I. — NO. IV. 12 



