220 



Prof. J. W. Judd. 



[Nov. 19, 



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 produced crystals of kaolin, 

 which form the basis of all the true clays, a large quantity of lime-, 

 magnesia-, iron-, soda-, and potash-salts with silica passing into solu- 

 tion ; while, in the latter case, the several minerals of the rock are 

 simply reduced to fragments of varying size and form. 



All the observations described in the present report are in entire 

 harmony with this explanation. The comparatively unaltered con- 

 dition 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 dissolved matter in the Nile- water, in spite of the enormous 

 concentration it must have undergone by evaporation — all point to 

 this same conclusion. 



In the estimates which have been made of the rate of subaerial 

 denudation in different parts of the globe, it has usually been assumed 

 that this action is similar to what is seen taking place in Europe and 

 in North America. But the observations detailed in this report prove 

 that in tropical districts, where little or no vegetation exists, and the 

 rainfall is sudden and torrential, the disintegration of rocks, though 

 not, perhaps, less rapid than in temperate climes, is different alike in 

 its origin and in its products. 



It has often been pointed out by chemical geologists, that metamor- 

 phic action could not have produced many of the schists and gneisses 

 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 whereby great masses of sedi- 

 mentary 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 and gneisses is one that presses for solution. 



Appendix. 



Examination of the Samples obtained during the Recent Borings in 



Egypt 



Portions of the several samples, varying in weight from 8 to 

 18 grams, were dried at 110° C, and then weighed. These portions 

 were, by careful levigation, separated into mud and sand, which being 

 dried at 110° C. and weighed, afforded the necessary data for calcu- 

 lating the percentage composition of each sample. 



One specimen of each sample of sand thus obtained was mounted 



