NATURE 



433 



THURSDAY, MARCH 12, 190S. 



THE FIRST NILE CATARACT. 



A Description of the First or Aswan Cataract of the 

 Nile. By Dr. John Ball. Pp. 121; with 13 maps 

 and plates, .-ind 20 illustrations in the text. (Cairo : 

 National Prinlinjj;- Department, 1907.) Price 200 

 millienies. 



AMONG the numerous valuable memoirs that have 

 issued from the Efjyptian Survey Department 

 under the energetic administration of Captain Lyons, 

 nor.e is likely to prove of greater general interest to 

 the public than this work of Dr. Ball. The easv 

 accessibility of .Aswan to visitors sojourning at Cairo, 

 the wealth of objects of antiquarian interest in :ts 

 neighbourhood, and the existence of that great en- 

 gineering feat — the Nile dam — ensure the result ot 

 •a constantly increasing stream of tourists to the 

 district; and although the English, German and 

 French guide-books to Egypt, published bv .Murray, 

 Baedeker and Hachette respectively, have such a well- 

 deserved reputation, yet the complete topographical 

 and geological survey of the district, made by so 

 competent an otTicial as the author, has enabled him 

 to supply many precise data and new observations not 

 hitherto accessible to the writers of these guide-books. 

 Dr. Ball, indeed, comes with excellent qualifications 

 to the task before him ; a good geologist, with special 

 knowledge of petrography, he is at the same time 

 skilled in surveying and engineering matters, while 

 the account which he gives of the literature bearing 

 on the district (pp. 15-20) shows that he has not been 

 unmindful of the importance of this branch of know- 

 ledge in connection with a country having such a 

 past as Egypt. 



-\t the outset, the author has to correct the popular 

 misconceptions concerning the nature of the Nile 

 "cataracts." He writes: — 



" There is nothing about a Nile cataract in any way 

 resembling Niagar.-i, nor even the Falls of the Rhine 

 at Schaffhausen. The total fall of the water-surface 

 at the First Cataract (between Philse and Elephantine) 

 is only about 5 metres in a length of about 9 kilo- 

 metres ; and although the greater part of the fall is 

 concentrated within a fraction of this total length, it 

 is only sufficient to give rise to rapids, and not to 

 a waterfall in the ordinary sense ot the term. The 

 obstruction to navigation offered by a Nile cataract 

 is in fact due, not so much to the velocity of the 

 water, as to the irregularity and conflicting nature 

 of the currents caused by the narrowness, winding 

 nature and rocky state of the channels." 



.Aswan was always a place of great importance. 

 Under its ancient name of "Syene," it is constantly 

 mentioned by the writers of antiquity, including the 

 prophet Ezekiel, and many of the Greek and Roman 

 authors. It formed the limit between Egypt and 

 Ethiopia (Nubia), and observations made on the 

 shadows cast by gnomons erected at Syene and 

 .Mexandria respectively were employed by the early 

 geographers in determining the size of the earth and 

 I he obliquity of the ecliptic. .Although Syene wa' 

 regarded as situated on the tropic of the Cancer, yet. 

 NO. 2C02, VOL. 77] 



as Dr. Hall points out, Aswan is really 37' 5;" (71 

 kilometres) north of the tropic; and the period at 

 which, by the secular variation of the obliquity, the 

 site of .Aswan coincided witli the tropic was about 

 3500 B.C. Besides the gnomon, there were deep 

 vertical wells sunk at Syrene, the bottoms of which 

 were illuminated by the sun at midday at the summer 

 solstice. These wells are mentioned bv many ancient 

 writers, including the geographers Strabo, Pliny, and 

 Ptolemy. 



The geological survey of this very interesting dis- 

 trict was, of course, facilitated by the numerous 

 excavations made during the construction of the great 

 dam. But, on the other hand, the non-existence of 

 any accurate topographical map of the district pre- 

 sented a difficulty which could only be got over by a 

 complete survey of the whole area round the cataract. 

 The line laid out for the dam by the engineers afforded 

 Dr. Ball an excellent base-line, and from this a net- 

 work of triangles was measured with a good theodo- 

 lite, the details being filled in with sufficient accuracy 

 by means of the plane-table. Heights were measured 

 from the mean Nile level by the theodolite. This 

 map, which is in si.x sheets, is a great improvement 

 en any previous one, and has been issued by the 

 Survey Department, its scale being j-gJoo, but a 

 reduced copy forms plate i. of the work before us. 



The geological formations present in the district as 

 shown by the geological map (plate ii.) are : — 



(3) Recent deposits, including those formed by the 

 wind (desert sands) and those deposited by the river 

 (Nile muds and sands). 



(2) Nubian sandstones and clays, which cap many 

 of the hills. 



(1) Metamorphic and igneous rocks, constituting the 

 foundation of the whole country. 



The observations of the author on the chemical 

 composition of the Nile muds and sands, and on the 

 nature and form of the mineral particles present in 

 them, are of great value and interest, and are illus- 

 trated by some excellent drawings, reproduced in 

 collotype in plate iii. It appears both from recent 

 analyses made in Cairo, as well as from the earlier 

 work of Hofmann, that the Nile- sands contain only 

 small amounts of the hydrated aluminium silicates 

 (kaolin, &c.), but consist mainly of finely comminuted 

 felspars and other minerals, but Utile altered. 



The work of the geological survey seems to have 

 demonstrated that the Nubian sandstones in this dis- 

 trict are wholly of Cretaceous age, although in the 

 Sinaitic Peninsula there are Carboniferous sandstones 

 of very similar appearance. 



In opposition also .to earlier statements made to 

 the contrary, it is shown that the igneous intrusions 

 are confined to the metamorphic rocks and that they 

 are all older than the Nubian sandstone. 



Aswan, or S^'ene. is of interest to petrologists from 

 the circumstance that a large and important class of 

 rocks derives its name from this locality. The name 

 " Syenite " was first applied to the granitic rocks 

 which were so familiar to the ancients from the cir- 

 cumstance that they were the materials of the great 

 ICgyptian monoliths (obelisks, statues, &c.). In 17SS, 



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