GEOLOGICAL HISTORY OF EGYPT AND THE NILE VALLEY. 325 
of limestone rock towering some 200 feet more than at 
present where it leaves its narrow channel at the apex of the 
Delta,* and numerous streams wearing back the limestone 
cliffs till they had assumed somewhat of the form and posi- 
tion they now occupy all round the plain of Lower Egypt. 
(2.) Pliocene Epoch. Geological history, especially during 
the later periods, presents us with frequent examples of 
depression of land succeeding elevation, and this is still 
going on over the surface of our globe. We are therefore 
not to be surprised when in working out the physical history 
of Egypt we find evidences of such oscillations of land and 
sea. The Miocene period was evidently one of elevation and 
erosion, as we have just seen; but it seems to have given 
place to one of depression of the land to an extent not 
indeed sufficient to cover the Libyan and Arabian tracts 
with sea-water, but to an extent of about 220 to 250 feet, if 
we take the present sea level asa datum. This inference is 
arrived at by the discovery of raised beaches, and terraces 
with sea-shells at levels such as are above stated at various 
points along the flanks of the hills bounding the Delta on 
both sides,f as well as m other parts of the region bordering 
the Levant. The positions of these terraces and beaches are 
indicated in Figs. 2 and 3. 
Before, however, entering upon the discussion of this last 
submersion, it may be well to refer to a remarkable episode 
in the zoological history of this part of the world, for the 
elucidation of which we are much indebted to Dr. Wallace. 
I refer to the great migration of Pachyderms, Carnivores, and 
other Mammalia from the Europasian continent into Africa, 
which took place about the time that we have now reached 
in the physical history of Egypt. 
Zoological Episode—Towards the close of the Plocene 
period the animals which ranged over the Europasian tract 
of continental land were largely representative of those of the 
* At what particular stage of the Miocene period the marine strat 
referable to this stage at Siuah (Ammon) and Genetfe were deposited it is 
difficult to say. Those of the former appear to have been formed within 
an arm of the sea stretching inland from the Gulf of Sidra. There are 
very few shells common to both deposits. 
+t Of course the existence of the shore beds here referred to might be 
accounted for by supposing the level of the outer sea (now the Mediter- 
ranean) to have risen 250 feet higher than at present ; but this is not a 
probable supposition, as it would involve the idea of a general rise of the 
whole outer ocean. As Lyell and others have shown, it is the land which 
ee and falls, while the ocean surface maintains nearly always its geodetic 
evel. 
