March i6, 1893] 



NATURE 



467 



flushes from behind, the rate goes up to fifteen days. 

 There is a very great difference in time and rate between 

 Green and Red Nile. The rise is 45 ft. at Aswan, 38 at 

 Thebes, and 25 at Cairo. 



From the data obtained at the gauges named which 

 have been kindly forwarded to me by Mr. Garstin, the 

 U.S. of State of the Public Works Department of Egypt, I 

 have ascertained that the average time taken by the flood 

 to travel now between Thebes and Memphis is about 

 nine days. Although the river bed is now higher than 

 formerly, the land around Thebes, according to Budge, 

 having been raised about nine feet in the last 1700 vears, 

 still the same elevation has taken place at Memphis, so 

 that no difference in the velocity of the stream would be 

 produced by this cause. 



The great difificulty experienced in understanding the 

 statements generally made concerning the Nile-rise 

 arises from the fact that the maximum flood is as a rule 

 registered in Cairo upwards of 40 days after the maxi- 

 mum of Aswan. 



For the following account of how this is brought about 

 I am indebted to the kindness of Col. Ross, R.E. :— 



"The behaviour of the flood at the Aswan gauge is as 

 follows : Between August 20 and 30 a good average 

 gauge of 16 cubits is often reached, and between August 

 27 and September 3 there is often a drop of about 30 

 centimetres. The August rise is supposed to be mostly 

 due to the Blue Nile and Atbara River. Between Septem- 

 ber I and 8 the irrigation officers generally look for a 

 maximum flood-gauge of the year at Aswan. This is 

 supposed to be the first flush of the White Nile. In the 

 middle of September there are generally two small flushes, 

 but the last twenty days of September are generally dis- 

 ■ tinctly lower than that of the first week. The final flush 

 of the Nile is seldom, later than the 21st to 25th 

 September. 



" All this water does not merely go down the Nile ; it 

 floods the different basins. The opening of these basins 

 begins from the south to the north. This operation is 

 generally performed between the 29th September and the 

 22nd October. The great Central Egypt basins are not 

 connected with the Nile for purposes of discharge into 

 the river between Asyut and near Wasta,or a distance of 

 395-90 kilometres = 305 kil. 



"The country in the middle or Central Egypt is broad, 

 and thus there is an enormous quantity of water poured 

 out of these basins into the lower reaches of the river 

 about the 20th October, which seriously raises the Nile 

 at Cairo, and in a good average year will bring the Cairo 

 gauge (at Rodah) up to the maximum of the year on or 

 about October 22, and hence it is that the guide books 

 say the Nile is at its highest in the end of Uctober. 



"A gauge of 164 cubits at Aswan while the basins are 

 being filled does not give more than 21 cubits at Rodah 

 (Cairo), but as the basins with a i6i gauge will fill by the 

 loth September, it follows that a 16^ to 16 cubit gauge at 

 Aswan will not give a constant Cairo gauge, as the great 

 mass of water passes by the basins and reaches Cairo. 

 Hence we have frequently the paradox of a steady or 

 failing gauge at Aswan showing a steady rise at Cairo. 



" It the gauge at Aswan keeps above 16 cubits to near 

 the end of September, the basin-emptying is much 

 retarded, as the emptying at each successive basin fills 

 the Nile above the 16 cubit level ; hence the lower halves 

 of the chains of basins do not flow off, and thus when 

 the great Middle Egypt basins are discharged, they do 

 not raise the Nile so much as they do when the last 

 half of September Nile is below 16 at Aswan. 



" In years like 1887 and 1892, which differ from each 

 other only in date of maximum gauge at Aswan, the 

 river, having filled the basins in 15 to 20 days instead 

 of m 25 to 30 days, comes down to Cairo in so largely 

 mcreased a volume that a really dangerous gauge of 25 

 cubits at Cairo is maintained for over a fortnight (the 

 NO. 1220. VOL. 47] 



average October gauge in Cairo is about 23 cubits), and 

 from September 10 to October 25 the river remains from 

 24 cubits to 254 cubits, and the Middle Egypt basins 

 discharge so slowly that the opening day is hardly trace- 

 able on the Cairo gauge. 



" In the 1878 flood, which was the most disastrous flood 

 possible, the river rose in the most abnormal fashion, and 

 on October 3 attained 18 cubits at Aswan. This breached 

 the Delta, and in addition so delayed the Upper Egypt 

 basins emptying from the reason before given that the 

 wheat w*as sown too late, and got badly scorched by the 

 hot winds of March and April." ^ 



J. Norman Lockyer. 



THE LANDSLIP A T SANDGA TE. 



T^HE causes of landslips are in general so well known 

 -•■ and the localities which are liable to them so clearly 

 defined on geological principles that when on Monday, 

 March 6, the public were startled by the news of a land- 

 slip at Sandgate, the probability would be that geologists 

 who knew the district would be by no means surprised, 

 more particularly as the locality of the catastrophe is in 

 the midst of a typical section shown in many of the text- 

 books, and the town itself gives its name to a subdivision 

 of the Cretaceous rocks. 



The event, however, does not appear to have been ex- 

 pected, and since it has happened conjectures as to its 

 cause have been numerous ; but the true explanation has 

 been wanting. 



The series of rocks which, in descending order, form 

 the country about Sandgate are the Folkestone beds, the 

 Sandgate beds, the Hythe beds, and the Atherfield clay. 

 Amongst these it is natural to look in the first instance 

 for the presence of clays, as the probable origin of a land- 

 slip, though very loose sands have also been known 

 to give way. The Folkestone beds are for the most part 

 sand and they are bound together by bands of grit. 

 Moreover, they are above the affected area. The Hythe 

 beds are likewise characterised by bands of hard lime- 

 stone, separated by calcareous sands. There are left the 

 Atherfield clay, whose nature is indicated by its name, 

 and the Sandgate beds. 



The rnost recent description of these is that of W.Topley, 

 F.R.S.,in the Comptes Rendus of the Congres Gdologique 

 International, 1888, in which they are briefly characterised 

 (p. 257) as ^^ Argil es vertes et sables." The same 

 writer's description of them in 1883 (quoted also by H. B. 

 Woodward in 1887) is somewhat different, but in his 

 " Geology of the Weald," 1875, they are said to consist of 

 dark clayey sand and clay, the total thickness being given 

 as 80 ft. In his more detailed description, however, Mr. 

 F. G. H. Price divides these 80 ft. into four parts, the lowest 

 20 ft. being all " clayey beds " (Proc. Geol. Assoc, vol. iv. 

 p. 554). In a still earlier account by Prof Morris {I.e. vol. 

 ii. p. 41) we have the following interesting statement : — 

 "The dark-greenish sub-argiUaceous sands, known as 

 the Sandgate beds, rise on the shore at a short distance 

 west of Folkestone. The low undercliff which skirts the 

 shore from Folkestone nearly to Hythe owes its origin to 

 the presence of these beds, which from their retention of 

 water and slight coherency of structure have caused the 

 frequent subsidence of the beds above." 



It would appear, then, that there are two possible 

 sources of the slipping— the Atherfield clay and the 

 clayey bands of the Sandgate beds, and thus much was 

 indicated at once by Mr. F. W. Rudler {Daily Graphic, 

 March 8). 



On a personal examination of the area the whole 

 history of the subsidence becomes clear enough. At 



' The modem Egyptians still hold to the old months for irrigation. 

 7 Tuba=Janii.iry 15 is commencement of wheat 'irrigation ; 30 Misra is the 

 last safe date for sowing mai/e in the Delta ; ist Tut is the date of regulat- 

 ing the bridges=September 8 in Upper Egypt. 



