GEOLOGICAL HISTORY OF EGYPT AND THE NILE VALLEY. 317 
Abyssinia, described by Dr. Blanford as underlying Jurassic 
limestone, belongs to this lower or Paleozoic division.* 
The ouly fossils in the Nubian sandstone proper are stems 
of plants; and it 1s probable that this great formation was 
deposited within the waters of a vast inland lake, occupying 
the greater portion of Northern Africa. The sediment of 
which it was formed was derived from the disintegration of 
the granites, crystalline schists, and quartzites of the 
Archean rocks, forming te continental land by which it was 
enclosed. Gradually this region subsided and was invaded 
by the waters of the Cretaceous ocean. 
(3.) Cretaceous and Eocene Limestones—The calcareous 
strata which overspread the Libyan Desert and the greater 
part of the Arabian Mountains belong to two formations: 
the Cretaceous and Eocene. The former is represented by 
the Chalk formation of England, France, and Belgium, and 
consists of soft white limestone and calcareous marls with 
characteristic fossils such as: Exogyra Overwegi, Inoceramus 
Crispi, Hippurites, Nautilus desertorum, N. Danicus, Sc. 
The Eocene limestone, generally known as the Nummu- 
lite limestone, from the occurrence of species of Nwmmulites, 
often in great numbers, is chiefly formed of solid beds of 
limestone with bands of chert. The fossils, which are nu- 
merous, are specifically different from those of the Cretaceous 
beds, though the strata are similar in character and origin, 
and are approximately conformable to each other. How are 
we to account for this remarkable change in the fauna of the 
two similar formations? Probably somewhat in this way: 
The Cretaceous limestones and marls were formed over the 
bed of the great ocean which spread its waters over all this 
region, from the Red Sea to the Atlantic, except where the 
mountainous tracts of ancient Paleozoic or Archean rocks 
rose above its surface. In the waters of this ocean the 
characteristic Cretaceous forms lived abundantly ; but, towards 
the close of the period, the bed of the ocean was slowly and 
generally elevated til it became either dry land, or very 
shallow. During this process of upheaval and desiccation 
the animals which had lived in the waters were necessarily 
destroyed; and when the region again partially subsided and 
became oceanic, new forms of living beings migrating from 
other oceanic areas, took possession of the waters, amongst 
which Foraminifera of the genus Mwmmulites were the most 
* W.T. Blanford, Geol. and Zool. of Abyssinia, p. 170. 
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