-98 



NA rURE 



{Jan. 2 8, 1886 



would be widely and closely connected with the people, 

 ;ind they would to all appearance — but who can speak 

 with certainty of the effects of any paper constitution — 

 form a vigorous and effective aristocracy. 



DEPOSITS OF THE NILE DELTA 

 T N a previous communication I referred to the probabi- 

 ■'■ lity that the lower portion of the Delta borings belongs 

 to the Pleistocene and Isthmian deposit which underlies 

 the modern Nile mud, and which has been recognised as 

 an important formation by nearly all geologists who have 

 studied the Nile Valley. 1 now propose to state shortly 

 some objections to the generalisations of the Report on 

 the Nile borings with reference to the causes assigned 

 for the comparative purity of the waters of the Nile, and 

 the character of its sediment, viz. that the former is due 

 to its flowing through a rainless country, and that the 

 latter is derived from the decay of rocks in this rainless 

 area, and this decay produced not by " chemical agencies," 

 but by "mechanical forces," namely, the "unequal ex- 

 pansion" of the constituent minerals under the influence 

 of heat and cold, aided by " the force of the wind." 



It is scarcely necessary to premise that neither the 

 water nor the mud of the Nile can be derived from the 

 rainless district through which the river llows, but from 

 the well-watered regions of interior Africa. The White 

 Nile, which carries scarcely any sediment, is a somewhat 

 constant stream, draining a country of lakes, swamps, and 

 forests. The Blue or Dark Nile and the Atbara drain 

 the mountainous country of Abyssinia, deluged with rain 

 in the wet season, and it is these streams, swollen by 

 violent inundations, that supply the Nile with its sedi- 

 ment, the quantity of fresh material carried into the 

 river below the confluence of the Atbara being very small, 

 as the results of the microscopic study of the sediment 

 sufficiently proves, and I can testify from my own exami- 

 nations of the Nile mud, that its composition, as stated 

 by Prof Judd, is essentially the same along the course of 

 the Nile as in the upper layers of the Delta borings, 

 tliough with some local differences in the fineness of tlie 

 sand and the proportion of argillaceous matter. Thus 

 both the water of the inundations and the material of the 

 alluvial deposit come from a region of copious rains, and 

 where decay of rocks may be supposed to proceed under 

 the ordinary conditions. 



What then is the cause of the freedom of the Nile 

 water from saline matter .' Simply its derivation from a 

 country of siliceousand crystalline rocks. If,instead of com- 

 paring it with the water of the Thames and other streams 

 draining sedimentary districts, it had been compared 

 with that of the lakes and streams of the Scottish High- 

 lands (by no means rainless districts) this would have 

 been apparent. Dr. Sterry Hunt has described and 

 referred to its true cause a fact of the same kind in the 

 case of the Ottawa and St. Lawrence. The former, rising 

 in a region of crystalline rocks, has little more than one- 

 third of the saline matter in solution that is found in the 

 latter, which drains principally a sedimentary country. 

 The proportions in 10,000 parts are, for the Ottawa, only 

 06116, and for the St. Lawrence, r6o55.i 



But it may be asked. Why in that case is the Nile mud 

 so deficient in kaolin } The answer is, that the current 

 of the river is sufficiently strong to wash out all the more 

 finely comminuted argillaceous matter and to cari-y it in 

 its turbid waters to the sea. In connection with this, 

 every voyager on the falling Nile must have observed 

 how the mud-banks arc constantly falling as they are 

 undermined by the river, and their material carried down 

 to be redeposited. This work goes on even more ener- 

 getically in the time of the inundations. Thus any given 

 quantity of sediment on its way from Abyssinia to the 



' Log.ln's ' Geology of Canad,-^," i?65, p. 565. 



Delta is lixiviated thousands of times, and necessarily 

 deprived of its lighter and finer constituents. 



But the quantity of kaolin need not originally have been 

 large. The older gneisses and schists do not kaolinise 

 after the manner of Cornish granites, but, when decom- 

 posed so as readily to crumble into sand, they still con- 

 tain much of their more refracting felspar in a perfect 

 state. 



These facts are farther illustrated by the agricultural 

 qualities of the Nile alluvium, as they have been ex- 

 plained by Schweinfurth and others. If the alluvial soil 

 were a stiff clay, it would be practically incapable of cul- 

 tivation in the circumstances of Egypt. If it were mere 

 quartzose sand, it would be hopelessly barren. It is, in 

 fact, an impalpable sand, highly absorbent of water, 

 crumbling readily when moistened, and containing not 

 merely quartz but particles of various silicates and of 

 aixUite and dolomite, which, though unaltered ■ when 

 under water, are gradually dissolved by the carbonic acid 

 present in the cultivated soil, yielding alkalies, phos- 

 phates, &c., to the crops. In connection with this, recent 

 microscopic examinations by Dr. Bonney of the old crys- 

 talline rocks of Assouan, which are probably similar to 

 those farther north, show that, like those of Canada and 

 Norway, they contain numerous crystals of apatite. 



As to the mechanical action of the heat of the sun on 

 crystalline rocks, any one who examines the polished 

 surfaces still retained by monuments which in Upper 

 Egypt have been exposed to this influence for thousands 

 of years, must be convinced that no disintegration of 

 this kind occurs. The only evidence of such actions that 

 1 have been able to find is the chipping of little circular 

 disks from the exposed sides of nodules of flint on the 

 surface of the desert. Granitic rocks decay, however, in 

 Egypt, as elsewhere, where they are exposed to moisture 

 from the soil, or where, as at Alexandria, they are subjected 

 to the influence of frequent rains and of saline particles 

 cai'ried from the sea. In this connection I may add that 

 Hague, in a jiaper in Scic/ict- on the decay of the New 

 York obelisk, shows that it had probably suffered (as, 

 according to Wigner, that in London has also done) from 

 atmospheric action before its removal from Alexandria, 

 and that this decay has been greatly increased by the 

 alternations of moisture and frost to which it is subjected 

 in New York.' 



At -Assouan, in a climate at present rainless, or nearly 

 so, I was surprised to find that the surface of the gneiss 

 and crystalline schists was in many places decayed to the 

 depth of several feet, so that it was impossible to obtain 

 fresh specimens except from the railway cuttings. This 

 may be due to the action of water and carbon dioxide 

 oozing through the ground, but is more probably a result 

 of more humid climatal conditions in former ages. 



1 hope at a future date to pursue these interesting 

 questions farther ; but in the meantime I shall be content 

 if it has been shown that Egypt owes the advantage of 

 pure, sweet water to the fact that it drinks of mountain 

 streams which the rainless character of its own climate 

 merely preserves from pollution by the drainage of the 

 Cretaceous and Tertiary beds, and that its rich alluvial 

 soil has not been produced by any mechanical action of 

 an exceptional nature, but by the ordinary atmospheric 

 agencies of denudation. 



These conclusions, as well as those stated in my 

 previous letter, respecting the depth of the modern 

 alluvium and its relation to the well-known Pleistocene 

 formation which underlies it, could be confirmed by the 

 testimony of most geologists who have studied the valley 

 of the Nile, and more especially of Lartet, Fraas, and 

 Schweinfurth. I hope that as now stated, how-ever im- 

 perfectly, they may suffice to induce the Committee 

 materially to modify its Report, or to postpone its publi- 



