324 PROFESSOR E. HULL, LL.D., F.R.S., F.G.8., ON THE 
One or two barrier ridges had, however, to be crossed in 
the case of the Nile, as in that of most rivers of any import- 
ance. The explanation of this singular physical fact is now 
fully understood. For a while these barriers, formed by the 
limestone escarpment where it crosses the valley north of 
the First Cataract, and by the sandstone and granite near 
Ipsambol below the Second Cataract, would prove formidable, 
and the waters probably accumulated behind them, till they 
rose above the lowest lip, and then poured down, continually 
deepening their channel as time went on. Once the channel 
was formed, it so remained,* and it must not be forgotten 
that the strata being somewhat elevated in a southerly direc- 
tion during the period of movement, was itself undergoing 
denudation, or lateral erosion ; and the rainfall being doubt- 
less abundant (as testified by the forests now silicified), 
the rocks were worn down by rain and river action, along 
the lines of outcrop of the strata. 
This deepening process, by water erosion, must have gone 
on pari passu with the uprising of the land. It is also necessary 
to suppose that the uprising continued until the land formed 
of Eocene limestone was elevated much higher relatively to 
to surface of the then ocean than is at present the case ;_ 
because, the erosion of the limestone beds in the valley of 
the river, and the Delta, must have gone on till the present 
floor of the Delta was reached, but which is now filled with 
sediment up to (or above) the level of the Mediterranean. 
The maximum depth of the floor is not yet ascertained, not- 
withstanding the boring experiments carried out by the 
Royal Society, 1854, under Mr. Horner, and more recently 
in 1886. If we assume the greatest depth to be 200 feet 
below the present surface of the alluvial plain, then this will 
give us approximately the amount of the additional rise of 
the land, relatively to the surface of the then outer sea. 
The greater relative elevation of the land during the 
Miocene period would result in giving the river a greater 
fall, and increasing the eroding power. Very little sediment 
would under these conditions be deposited; and thus, at the 
close of the Miocene epoch, we may contemplate the Nile as 
rushing along its rocky bed towards the outer sea, its banks 
* It will be understood I do not refer to the changes of position of the 
Nile within the limits of its alluvial plain. ; 
+ This will be evident when it is recollected that erosion could only go 
on above the sea level; for, under the opposite conditions of the land 
deposition of sediment, and not erosion, would be in progress. 
