Bocks in the Nile Valley^ in Nubia. 138 



In the assumption of an excavation of tho bed of the river, we 

 have no small amount of wear to deal with, for the distance from 

 Semne to Assuan, following the course of the river, is not less than 

 250 miles; and if, as Professor Lepsius supposes, the excavation 

 extended to Meroe, we have a distance, between that place and 

 Assuan, of not less than 600 miles. 



Although these records of a former high level of the Nile at 

 Semne had not been noticed by any traveller prior to Professor 

 Lepsius, we may rest fully assured of the accuracy of his statements, 

 from the habitual care and diligence, and the established character 

 for fidelity, of the observer. The silence of other travellers may be 

 readily accounted for by this, that none of them appear to have re- 

 mained more than a very short time at this spot — not even the diU- 

 gent E-ussegger — whereas we have seen that Professor Lepsius 

 passed twelve days in tho examination of this gorge in the Nile 

 Valley. 



The theory of a lowering of the bed of the river by wearing, in- 

 volves two main considerations, viz., the power of the stream, and 

 the degree of hardness of the rocks acted upon. The power depends 

 upon the volume and velocity of the river — the velocity on its 

 depth, and the degree of inclination of the bed : the hardness of the 

 rocks we can form a tolerable estimate of when we know their na- 

 ture. To judge, therefore, of the probability of the hypothesis of 

 Professor Lepsius, we must inquire into the physical and geological 

 features of the Nile Valley, in Nubia. 



In the observations I have now to offer, my information has been 

 derived of course entirely from the works of other travellers, parti- 

 cularly those of Burckhardt, Riippell, and Russegger,* and especi- 

 ally the latter, who travelled in Nubia in 1837 ; for he not only 

 enters far more into the details of the natural history of the country, 

 but he is the only traveller in Nubia who appears, from previous 

 acquirements, to have been competent to describe its natural history 

 with any degree of accuracy — I refer more particularly to the physi- 

 cal and geological features of the country. Besides full descriptions 

 in his volumes, he has given a geological map of Nubia, and also 

 several sections, or what may more properly be called vertical 

 sketches — a term that would, perhaps, be a more appropriate designa- 

 tion for all sections that are not drawn to a true scale, or at least 

 when the proportion of height to horizontal distance is not stated. 



The Physical Geography of Lower Nuhia,'\ 

 Russegger informs us,J that he believes he was the first traveller 



* Reisen in Europa Asien und Afrika, in der Jahren 1835, bis 18il. 

 Stuttgart 1841-1846. 



t With reference to the object of this paper. 

 X Ileisen, Bd. ii., 545. 



