Geological Description of the First Cataract, Upper Egypt

Abstract

Abstract A t the First Cataract the Nile flows over crystalline rocks consisting principally of quartz, felspar, and hornblende, combined in various, proportions, and then appearing under the forms of syenite, greenstone, hornblende, and mica-schists, or else occurring in separate masses. In the bed of the river the surface of the harder portions of these rocks is beautifully polished. The whole district is traversed by dykes of greenstone, of which the prevailing direction is E. and W. The crystalline rocks forming the bed of the river are overlain by a sandstone, sometimes coarse and gritty, and at other times fine-grained and compact. The prevailing colour is light yellow; but in places it is dark-purple and even black, owing to the presence of iron. As yet no organic remains have been discovered in it. This sandstone rests on the uneven surface of the syenite, in slightly inclined strata dipping N.N.E. It is nowhere altered at its junction with the syenite, nor is it anywhere penetrated by dykes. To the eastward of the First Cataract is a wide valley, commencing opposite the Island of Philæ, and joining the Nile valley again about three miles below Assouan. Through this valley the Nile may have formerly flowed, as freshwater shells and deposits of Nile-mud are found at a considerable height above the present level of the river. To the westward of the First Cataract the crystalline rocks disappear below the sandstone, and the country is almost entirely covered with sand of a rich yellow colour, composed of fine rounded grains of quartz.