214 



Prof. J. W. Judd. 



[Nov. 19, 



varying proportions, of blown sand and alluvial mud. I can find no 

 evidence to support the suggestion made by Sir J. W. Dawson, F.R.S. 

 (see Colonel Maitland's letter of 21st January, 1884, and " G-eol. Mag.," 

 Dec. 3, vol. i, p. 292) from a hasty examination of the specimens, 

 that " at a depth of 30 or 40 feet the alluvial mud rests on desert 

 sand;" on the contrary these borings, like those of older date, show 

 that the deposits of the Kile Valley consist of a succession of diiferent 

 beds in some of which sand, and in others mud, forms the predominant 

 constituent.* 



The chemical composition of the Nile-mud has been investigated by 

 many chemists — by Regnault in 1812, by Lassaigne in 1844, by 

 Lajouchere in 1850, by Payen and Poinset in 1850, and by Houzeau 

 in 1869. At the request of Mr. Horner, Messrs. Johnson and Brazier 

 undertook a series of careful analyses of the muds and the sands of 

 the Nile- Valley deposits in Dr. Hofmann's laboratory, and the results 

 which they obtained are published in the " Philosophical Trans- 

 actions," vol. 145 (1855). 



But hitherto, so far as I am aware, no detailed investigation of the 

 microscopical characters of the deposits has been undertaken, and to 

 this investigation I have therefore devoted my special attention. The 

 methods of examination were determined upon after consultation with 

 Professor T. Gr. Bonney, F.R.S., P.G.S., to whose advice and assis- 

 tance I am greatly indebted; the preparations were made in the 

 Geological Laboratory of the Normal School of Science and Koyal 

 School of Mines. The general results attained by this study of the 

 Nile- Valley deposits are as follows. 



The sands, when separated from the mud by washing, are found to 

 be made up of two kinds of grains, the larger being perfectly rounded 

 and polished, while the smaller, on the contrary, are often subangular 

 or angular. 



The larger and well-rounded grains may be described as micro- 

 scopic pebbles ; their surfaces are most exquisitely smoothed and 

 polished, and their forms are either globular or ellipsoidal. In size 

 they vary greatly, being occasionally as large as a small pea. They 

 only very occasionally exhibit traces of deposits of iron-oxides upon 

 their surfaces. The great majority of the well-rounded grains consist 

 of quartzose materials. 



* [The deep boring at Rosetta, of which an account has recently been sent to the 

 Royal Society by Colonel Maitland, R.E., shows that alluvial mud and clay occurred 

 to the depth of 33 feet ; thence to the depth of 94 feet various sands and clays 

 are recorded ; from 94 feet to 123 feet " hard clay in lumps " was found ; then 

 various sands and clays to 143 feet ; the last 10 feet of the boring was in coarse 

 sand and pebbles. A fragment of red granite, with undoubted marks of human 

 workmanship upon it, is said to have been obtained at a depth of 79 feet 4 inches. — 

 J. W. J., 18th December, 1885.] 



