4 o6 SCIENCE PROGRESS. 



near the oasis of Siwa, Zittel discovered nummulitic beds of 

 a different character. They are only some 10 m. thick, 

 and consist of white and brown limestones, made up almost 

 entirely of fragments of shells. Amongst the fossils which 

 occur in them are Nummulitis intermedins, and N. Fichteli, 

 which belong to De la Harpe's seventh nummulitic horizon 

 (including uppermost Eocene and lower Oligocene) ; but 

 along with these are also found such typically Eocene 

 forms as N. Guettardi, etc. The sea-urchins belong mostly 

 to species which occur also in the Mokattam stage. The 

 molluscs, according to Mayer-Ey mar's determination, in- 

 dicate a Bartonian age. 



North of Mokattam rises the hill which is called Jebel 

 Ahmar. It consists chiefly of a brownish sandstone 

 rendered very tough by a siliceous cement. This is the 

 Jebel Ahmar Sandstone, or Nicolia Sandstone of Zittel, 

 and it rests upon the upper beds of the Mokattam stage. 

 Silicified stems of trees are not uncommon in it, but other 

 fossils are rare. Unger (50), however, has recorded the 

 presence of a Helix, and of some almost indeterminable 

 fresh-water shells. More recently Mayer-Eymar has dis- 

 covered near its base a bed containing fresh-water shells. 

 These he at first referred to the Upper Tongrian (17) i 1 but 

 on a closer examination he has found them to belong to 

 the Lower Tongrian (18), and he looks upon the Jebel 

 Ahmar Sandstone itself as Upper Tongrian. The usual 

 view hitherto has been that the sandstone is Miocene ; but 

 the evidence for this was verv weak. 



Whatever the age of the Jebel Ahmar rock may be, it 

 is generally agreed that it is the same as the sandstone 

 which occupies so large a part of the desert west of Cairo. 

 It extends from the Wadi Natrun southwards to about lat. 

 29 N., and a small area occurs also on the east side of the 

 Nile. In some places the fossil trees are so abundant as 

 to have suggested the name of " Petrified Forests ". Alony; 

 its southern margin it overlies a bed of oysters (O. Fraasi 

 and 0. Cloti\0. Clot- Bey 7']) of Upper Mokattam age (16) ; 



1 ■ 



I'he Tongrian is one of the divisions of the Oligocene. 



