Sh' Wm. Dawson — N'otes on the Geology of Egypt. 441 



Eocks of the character above described might of course admit of 

 different interpretations as to their relations and origin ; but as seen 

 on the ground, they undoubtedly have the aspect of an overlying, 

 unconformable stratified formation, and their crystalline character 

 must be due to the conditions of their formation, and not to any 

 subsequent mechanical action. They are, therefore, to be regarded 

 as igneous or aqueo-igneous deposits. 



Above the Cataract the river passes through a gap in the rocks 

 above described, between the Island of Biggeh and the eastern shore, 

 and runs over the older series, the granitic dykes of which project in 

 prominent masses above the softer schists, as may be seen in the 

 rocks of Konosso, and the eminence on the west end of Philge. 



In cutting back its channel, the Nile must originally have formed its 

 First Cataract at the ridge of Silsilis, about forty miles below Assouan, 

 and its waters were then dammed up so as to flood much of the 

 river valley between Silsilis and the present site of the First Cataract. 

 At this time the Nile probably flowed along the old channel east of 

 Assouan ; but so soon as the channel was cut back thi'ough the Silsilis 

 ridges, it would rapidly extend southward through the softer beds to 

 Assouan, and on reaching this place, the river would begin to remove 

 the Nubian sandstone capping the crystalline rocks at the site of the 

 present Cataract. This process would seem to have disclosed an 

 ancient break or soft portion in the underlying formation, enabling 

 the present channel to be cut, and this has been done mainly by 

 removal of the sandstone and of loose fragments of the second 

 crystalline formation above referred to. 



In so far as the locality at the First Cataract is concerned, we have 

 no precise measure of age for the crystalline rocks. There is reason 

 to believe that in the range of similar formations extending north- 

 wards between the Nile and the Red Sea, beds occur of ages inter- 

 mediate between those of the rocks of the Cataract and the Nubian 

 sandstone ; but the precise ages of these intermediate rocks are as 

 yet uncertain. In these circumstances mineral character becomes 

 our only guide. But this is by no means uncertain in its testimony. 

 The schists and gneisses of the older Assouan series are identical in 

 mineral character with those of the Grenville series of the Canadian 

 Laurentian, and they have already been compared by Drs. Liebisch 

 and Hochstetter with the rocks of the same age in Scandinavia.^ In 

 like manner, the second or overlying series has two points of simi- 

 larity with the felsitic series found in America to occur at the base 

 of the Huronian, which has been named by Hicks the Arvonian 

 series, and to which the Swedish geologists have given the name 

 Halleflinte. I think, therefore, we may be justified in regarding 

 these old crystalline rocks as African representatives of the Lauren- 

 tian and one of the succeeding crystalline formations, and of course 

 the same conclusion would apply to the wide extent of similar rocks 

 in this part of Africa, and which recur in the peninsula of Sinai. 



1 Geol. Society of Germany, Jahrbuch, 1877. I have placed a suite of specimens 

 in the hands of Prof. Bonney, F.R.S., who I hope may at some future time report ou 

 their precise lithological characters. 



