598 PROF. J. W. GREGORY ON [Nov. I (Jit, 



They probably represent part of the Lower Eocene or Libyan Series 

 of Egypt. The chert-bearing Apollonia Limestones also yielded 

 no determinable molluscs or echinoids, but their infraposition to 

 the Derna Limestone shows that they correspond to the chert- 

 bearing Upper Libyan Limestones of Egypt. They are succeeded 

 above by the Derna Limestone, which is shown by its beautifully 

 preserved foraminifera to be the equivalent of the Lower Mokattam 

 Limestone in Egypt, and is therefore of Middle Eocene age. The 

 Slonta Limestones, a stratified series above the Derna Limestone, 

 according to Mr. Chapman's identification of the fossils, are also 

 Middle Eocene. But Mr. Newton reports that the mollusca are 

 Priabonian, which is regarded either as the uppermost series of the 

 Eocene or the lowest of the Oligocene. 



The evidence of the echinoids agrees with that of the mollusca. 

 One of the most characteristic beds in the Slonta Series yielded 

 many specimens of Echinolampas ; but I have been unable to 

 recognize among them any specimen, either of Echinolampas cramerl 

 or of E. africanus, the characteristic Upper Mokattam species : 

 the Echinolampids certainly have Upper rather than Middle Eocene 

 affinities. 



The supposition that the Slonta Limestones may represent the 

 Upper Mokattam stage is, on the other hand, supported by some 

 stratigraphical considerations. Thus, at or near the top of the Slonta 

 Limestones is a widespread horizon containing reef-building corals. 

 The specimens obtained were casts, and they have not yet been 

 closely examined ; but the development of the reef -corals at this 

 horizon must be clue to a shallowing of the sea in Cyrenaica, and this 

 change may have been contemporary with the growth of the reefs 

 of Orbicella and other corals in the Upper Mokattam Beds of Egypt. 



The Upper Mokattam Beds in Egypt are succeeded by fresh- 

 water and terrestrial deposits, and the only quartz-pebbles found 

 in Cyrenaica, except along the coast, were obtained at Gasr el 

 Migdum, in a bed a little above the coral-reef horizon. This pebble- 

 bed, and the marl containing flattened stem-like fragments that 

 forms the springs at Cyrene and Messa, may represent a strati- 

 graphical break corresponding to the Egyptian freshwater beds; 

 and there is clearly a gap in the succession at or a little above 

 this horizon, since no representative of the Lower Oligocene (Ton- 

 grian) was found. The Slonta Beds are succeeded by the Cyrene 

 Limestone, which is referred to the Upper Oligocene (Aquitanian). 



If the reef-coral limestone in Cyrenaica is on the same horizon 

 as that of Egypt, then the Slonta Beds would be Middle Eocene, 

 and the gap in the succession in Cyrenaica would include both the 

 Upper Eocene and the Lower Oligocene. 



That the Derna Limestone is Middle Eocene is undoubted. It 

 is overlain by a hard or rough-weathering limestone, some layers 

 of which contain so many echinoid-plates and spine-fragments as 

 to suggest for it the name of the Echinoid Limestone. Some 

 of the plates came from regular echinoids, but I unfortunately 

 did not collect sufficient for generic determination. Above 

 this Echinoid Limestone is a soft white marl containing Fibularia 



