368 MR. T. BALL ON SOME ERODED 



27. On some Eroded Agate Pebbles /ro9?i the Sotjdan-. By Y. Ball, 

 Esq., M.A., r.It.S., E.G.S., Director of the Science and Art 

 Museum, Dublin. (Bead March 28, 1888.) 



Amoxg a large number of pebbles and rock specimens collected by 

 Surgeon-Major Greene in the Soudan, and recently presented by 

 him to the Science and Art Museum in Dublin, I found that the 

 majority of the former were of very similar character to the Agate 

 and Jasper pebbles derived from the basalts of India. Upon closer 

 examination I observed that a certain proportion of them are eroded 

 in a manner unlike anything I have ever noticed in India, though, 

 if the cause of erosion is what I believe it to have been, it is most 

 probable that similarly eroded pebbles would be found in certain 

 localities there also. Inferentially we may conclude, from the 

 nature of these pebbles, that they originally came from a region in 

 which basaltic rocks occur to a considerable extent. That this 

 conclusion is of no little importance will presently be seen. 



Throughout India, wherever there is deficient subsoil drainage on 

 the one hand, or excessive evaporation and limited rainfall on the 

 other, saline matters make themselves apparent, either in saturated 

 subsoil solutions, or by crystallizing out in the soil itself, in the 

 latter case, during wet seasons, being sometimes the cause of saline 

 lakes and pools. 



The nature of these salts varies with that of the minerals 

 constituting the rocks from which they have been derived ; and in 

 reference to the distribution of the various salts of sodium, potas- 

 sium, and magnesium, extensive observations have been made, both 

 in India and America, in connexion with the injurious effects upon 

 cultivation of these salts, which in India are commonly spoken of 

 collectively by the native term reJi. Considerable light, too, has 

 been thrown upon the question in connexion with the economic 

 utilization of these salts. A resume of this information will be 

 found in my ' Economic Geology of India,' and it need not be further 

 discussed here, save as regards the character of the salt, which is 

 most abundantly found in basaltic regions. In the Lonar lake in 

 Berar, which occupies a large hollow in the basalt, carbonate of soda 

 is deposited in such abundance from the water, which becomes super- 

 saturated dming the heats of summer, that it has some considerable 

 economic importance. Incrustations of carbonate of soda are also 

 met with even on the banks of streams traversing basaltic regions, 

 and though other salts are not altogether absent, it is this one 

 which is most abundant. 



Now whatever may be the potency of other alkaline salts, — in- 

 cluding borax — in reference to the solution of silica, there can, I 

 believe, be no doubt as to the great effect which a supersaturated 

 solution of carbonate of soda would have on silica, especially when, 

 as we may suppose was the case with these pebbles, they were 



