Dec. lo, 1885J 



NA TURE 



143 



abundant, while plagioclase felspar is comparatively rare. With 

 the rounded grains of quartz and felspar, a few examples of horn- 

 blende and other minerals, including jade, also occur. 



But far greater is the number of mineral species represented in 

 the smaller subangular and angular sand-grains. In addition to 

 the minerals already mentioned, I have recognised several 

 varieties of mica, augite, enstatite, tourmaline, sphene, dichroite 

 (cordierite), zircon, fluorspar, and magnetite. 



The muii'n a much more difficult material to study the mineral 

 characters of than the sand, owing to the extreme minuteness 

 of its particles. It is a ver striking fact, however, that kaolin, 

 which constitutes the predominant ingredient of clays, appears to 

 be almost absent from these Nile-muds. Chips and flakes of 

 quartz, felspar, mica, hornblende, .and other minerals, can be 

 re.adily recognised, and it is often evident that the unaltered 

 particles of such minerals make up the greater part, if not the 

 whole mass, of the fine-grained deposits. The mineral par- 

 ticles are, of course, mingled with a larger or smaller proportion 

 of organic particles. Frustules of Diatomncav occur in these 

 muds, as was pointed out by Ehrenberg, but unless special 

 precautions were observed in collecting the samjjles it would be 

 unsafe to draw any deductions from their ])resence. 



The striking peculiarities of these sands and muds of the Nile- 

 Valley appear to be capable of a simple explanation. In 

 countries where rain falls and vegetation abounds, water charged 

 with carbonic acid is constantly tending to break up the com- 

 pound silicates; the silicates of the alkalies and the alkaline 

 earths being decom])osed and their constituents removed in 

 solution, while the silicate of alumini becomes hydrated, and is 

 carried away in suspension by water in the form of kaolin. In this 

 way, the felspars and nearly all other compound silicates are 

 affected to .such an extent that in most granitic and meta- 

 morphic rocks they show evidence of extensive " kaolinization," 

 while the clays derived from them are made up for the most part 

 of crystalline plates of kaolin. Bat in a rainless country, 

 like Northern Africa, none of these .agencies will operate, and 

 the disintegration of the solid rocks is' effected by mechanical 

 means ; the most potent of these mechanical ;igents are the heat 

 of the sun, causing the unequal expansion of the minerals which 

 buildup the rocks, and the force ofthe wind, producing constant 

 attrition ofthe disjoined particles. 



This being the case, it will be readily understood that the 

 coarser sand-grains will include felsp.ar and other minerals in a 

 nearly unaltered condition, while in countries where the chemical 

 agents of the atmosphere come into play, such particle; would 

 be more or less completely converted into kaolin. In the same 

 way the mud, instead of consisting of scales of kaolin originating 

 from chemical actim, will be formed of particles of the chemi 

 cally unaltered niinerals reduced to the finest dust by purely 

 mechanical agencies. 



The chemical analyses which have been made of these Nile 

 muds entirely support these conclusions. Instead of containing 

 a considerable proportion of combined water, as do all the 

 ordinary clays, their composition is that of a mixture of anhy- 

 drous minerals. 



But there is fortunately a kind of evidence, derived from 

 chemical analysis which is of the greatest value from its hearing 

 on the questions we are now discussing— that, namely, which is 

 derived from a study ofthe composition of the Nile-waters. 



It must be remembered that the Nile is a river of a very 

 peculiar and exceptional character. The last tributary which it 

 receives is the Atbara, which falls into it in lat. 17° 38' N. ; from 

 that point to its mouth, in 31° 25' N. hat., the river does not 

 receive a single afHuent ; for a distance of 1400 miles it acquires 

 no fresh supply of water except what is Ijrought to it by super- 

 ficial torrents after heavy rains in Lower Egypt. It has been 

 cleariy demonstrated that, after receiving the Atbara, the Nile 

 undergoes a continual diminution in volume in its course 

 through Egypt. This is no doubt in part due to percolation of 

 the water through the delta-deposits, and in part to the water 

 being drawn off in canals for purposes of irrigation ; but a large 

 part of this diminution in volume must certainly be ascribed to 

 the great evaporation which must be going on from the surface 

 of the river during the last 1400 miles of its course. 



Although we shall not be able to calculate the exact loss of 

 the Nile by evaporation in the course of 1400 miles through 

 one of the hottest and driest regions of the globe, yet we cannot 

 doubt that this loss is enormous. Now the effect of this 

 constant evaporation must be to concentrate the saline matters 

 held m solution, and we might therefore anticipate that the 



waters of the Nile in Lower Egypt would contain an excep- 

 tionally high jiercentage of saline matters in solution. 



But what are the actual facts of the case ? 



According to the analyses of Dr. C. Meymott Tidy, the Nile 

 contains only a little more than one-half of the proportion of 

 soluble materials whicli exists in the Thames, the Lea, the 

 Severn, or the Shannon ! 



A little consideration will show, however, that this startling 

 and seemingly anomalous result is capable of simple and easy 

 explanation. The substances dissolved in the water of rivers is 

 of course derived from the materials composing the rocks of the 

 river-basin, through the action of w.ater holding carbonic acid or 

 other acids in solution. 



Hence we are led by the study of the composition of the Nile 

 water to the same conclusion as was reached by the study of 

 microscopical characters of the muds and sands of the delta, 

 that while in the rainy districts of the temperate zones the disin- 

 tegration of rocks is mainly effected by chemical agencies, in the 

 rainless areas of the tropics the same work is almost exclusively 

 effected by mechanical forces. 



The products of these two kinds of action are, however, 

 essentially different. In the former case we have formed 

 crystals of kaolin, which constitute the basis of all the true clays, a 

 large quantity of lime, magnesia, iron, soda, and potash salts with 

 silica passing into solation ; while, in the latter case, the several 

 minerals of the rock are simply reduced to fragments of varying 

 size and form, and but little matter passes into solution. 



The whole of the observations described in the present report 

 are in entire harmony with this explanation. The compara- 

 tively unaltered condition of the felspars and other complex 

 silicates in the sands ; the absence of kaolin from the muds, 

 and the presence of the chips and flakes of the unattacked 

 minerals in the muds ; and finally the small quantity of dis- 

 solved matter in the Nile-water, in spite of the enormous con- 

 centration it must have undergone by evaporation — all point to 

 this same conclusion. 



In the estimates which have been made of the rate of sub- 

 aerial denudation in different parts of the globe, it has usually 

 been assumed that this action is similar to what is seen taking 

 place in our own country and in North America. But the 

 observations detailed in this report prove that in rainless 

 tropical districts, where little or no vegetation exists, the dis- 

 integration of rocks, though not, perhaps, less rapid than in 

 temperate climes, is different alike in its causes and in its 

 products. 



It has often been pointed out by chemical geologists that 

 metamorphic action could not have produced many of the schists 

 from sedimentary rocks, for the former are rich in potash, soda, 

 and other materials which have been dissolved out from the 

 latter during the disintegration of the rock-masses from which 

 they were derived. The recognition of a kind of action where- 

 by great masses of sedimentary materials can be produced, rich 

 in those substances which are usually removed in a state of 

 solution, is not destitute of interest at the present time, when 

 the question of the origin of the crystalline schists is one that 

 presses for solution. 



Paris 



Academy of Sciences, November 30. — M. Jurien de la 

 Graviere, Vice-President, in the chair. — The Vice-President 

 announced the death of the President, IM. Henri Bouley, who 

 died on the morning of the same day. The speaker referred in 

 warm terms to the career of M. Bouley, his entire devotion to 

 science, and the courage with which, although suffering from a 

 fatal disease, he continued to the last to fulfil the duties of his 

 office. — Obituaiy notices of M. Bouley : by M. Herve Mangon, 

 in the name of the Academy of Sciences ; by M. A. Milne- 

 Edwards, in the name of the Natural History Museum ; by 

 M. A. de Quatrefages, as Vice-President of the Acclimatisation 

 Society ; and by M. Fremy, Member of the Academy. — As 

 a mark of respect for its late President, the public meeting of 

 the Academy was immediately adjourned. 



Berlin 

 Physiological Society, October 30.— Prof. Zuntz spoke on 

 the npncea of the foetus and the cause of the first respiration, 

 setting forth the pre-ent .state of the question, and then passing 

 to consider the assertion of Prof Preyer, who, by experi- 

 ments on r.abbits and guinea-pigs, sought to prove that it was 

 not the change in the gas of the blood which was 

 the cause of the first respiration, but a stimulus exercised 



