21(5 



Prof. J. W. Judd. 



[Nov. 19, 



The mud is a material of which it is much more difficult to study 

 the mineral characters than the sand, owing to the extreme minute- 

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

 which constitutes the predominant constituent of clays, appears to 

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

 of quartz, felspar, mica, hornblende, and other minerals can be readily 

 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 particles are of course mingled with 

 a larger or smaller proportion of organic particles. Frustules of 

 Diatomacece occur in these muds, as was pointed out by Ehrenberg, 

 but unless special precautions were observed in collecting the samples 

 it would be unsafe to draw any deductions from their presence. 



In the case of the Tantah boring we find another kind of material 

 besides the sand and mud. Almost all the samples from this boring 

 contain angular fragments of an apparently tufaceous limestone. In 

 addition to this we find minute sand-grains cemented by calcareous 

 matter into small pellets of various sizes, and the particles of mud 

 united in the same way are thus converted into argillo-calcareous 

 concretions. In a sample taken from a depth of 70 feet in the Tantah 

 boring, a number of irregular masses of this argillaceous limestoue 

 occurred; some of these were nearly 1 inch in diameter, and were at 

 first supposed to be pebbles. On examination they were found to 

 contain — 



Calcic carbonate 77*3 



Argillaceous matter 22'7 



lOO'O 



There can be no doubt that sources of this calcic carbonate must 

 exist near Tantah, probably in the form of calcareous springs. 



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 frequently and vegetation abounds, water charged with carbonic 

 acid is constantly penetrating into the rocks and breaking up the 

 compound silicates of which they are composed ; the silicates of the 

 alkalies and the alkaline earths being decomposed and their con- 

 stituents removed in solution, while the silicate of alumina 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 

 metamorphic rocks they show evidence of extensive " kaolinization," 

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

 of the crystalline plates of kaolin. But in dry and barren tracts, like 

 portions of Northern Africa, none of these agencies will operate, and the 



