GEOLOGY. 255 



the second view, the overlying sandstones may have contained 

 small quantities of copper-bearing minerals, in a similar manner to 

 the Kupferschiefer and other Permian and Triassic rocks in 

 Europe. Supposing the trapjiean rocks to have been percolated 

 by solutions cai'rying the products of the altei-ation of such min- 

 erals, it was suggested that the reduction to the metallic state was 

 mainly produced by the action of substances containing protoxides 

 of iron, which, by higher oxidation, have given rise to the dark-red 

 color and the eartliy ochreous substances found in the vein-matter. 

 The causes producing the metalliferous deposits in the trap were 

 stated to have evidently acted throughout the whole system, and 

 the absence of copper from the compact beds is probably rather 

 due to the absence of cavities fit for the reception of such masses, 

 than to any difference in chemical composition. 



GEOLOGICAL DESCRIPTION OF THE FIEST CATARACT, 

 UPPER EGYPT. 



At the first cataract, the Nile flows over crystalline rocks con- 

 sisting principally of quartz, felspar, and hoi-nblende, combined in 

 various proportions, and then appearing under the forms of syen- 

 ite, 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 direc- 

 tion is E. and W. The crystalline rocks forming the bed of the river 

 are overlaid by a sandstone, sometimes coarse and gritty, and at 

 other times fine-grained and compact. The prevailing color 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 no- 

 where altered at its junction with tlie syenite, nor is it anywhere 

 penetrated by dykes. To the eastward of the first cataract is a 

 wide valley, commencing opposite the Island of Philse, and join- 

 ing the Nile valley again about tliree miles below Assouan. 

 Through this valley the Nile may have formerly flowed, as fresh- 

 water sliells 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 sand- 

 stone, and the country is almost entirely covei'cd with sand of a 

 rich yellow color, comjiosed of fine rounded grains of quartz. — 

 J. C. Hawkshaw, in Header. 



ON THE ORIGIN OF PRAIRIES. 



The origin of prairies has recently formed the subject of three 

 papers in the " American Journal of Science," 1865, by Prof. 

 Winchell, Mr. Lesquereux, and Prof. Dana, in the order men- 

 tioned. Prof. Winchell believes that the prairies are of lacustrine 

 origin and of post-glacial date ; that all seeds contained in these 

 lacustrine deposits would perish, and that the vegetation which 



