1840.] 



of the Great Basaltic District of India. 



89 



it. On analysis, the salt was found to consist in 100 parts : of carbonic 

 acid, .3« ; soda, 40-9 ; water, 20-6 ; insoluble matter, -o, and a trace of a 

 sulphate. This nearl}' corresponds to the composition of the trona or 

 striated soda from the lakes of Fezzan^ examined by Mr. R. Phillips* ; 

 but approaches somewhat nearer to the equivalent numbers of the ses- 

 quicarbonate established by that analysis, which is to be ascribed to the 

 grearer purity of the Louar salt. The water of the lalce contained be- 

 sides, a little potash, muriate of soda 29 grains, sesquir-arbonate of soda 

 4-2 nearly, and sulphate of soda T of a grain in 1000 grains of the water. 

 Ko lime could be detected in it, nor did I discover any magnesia. These 

 facts confirm FJerthoUet's theory of the formation of carbonate of soda 

 in the natron lakes of Egypt, viz. that of a mutual decomposition of the 

 muriate of soda and carbon ^te of lime when in a pasty state. 



The .striated soda of Fezzan ^nd of this lake, containing half an equi- 

 valent more of carbonic acid than can be furnished by carbonate of lime, 

 readers some modification of his theory necessary and the most proba- 

 ble explaiiation appears to be, that the carbonic acid by which the lime 

 is held in solution iu the mud, furnishes the acid, and perhaps indicates 

 the existence of an unstable sesquicarbonate of that substance ; which 

 is not improbable from other facts in the history of the union with that 

 acid. This was suggested by my friend Captain Smith, of^the Z\Iadras 

 Engineers, and explains the circumstance sufficient!}-. 



Like most points in physical science, these observations have other 

 and more extensive applications than that of affording an explanation of 

 the production of natron in this lake. Carbonate of soda is extensively 

 distributed over the surface of the soil in various countries, more espe- 

 cially in the basaltic, portion of Central and Western India, and in the 

 limestone districts of the south ; the real relations of which have been a 

 subject of discussion amongst Indian geologists, and inferences unsup- 

 ported by fiact have been drawn from it. In all the places where I have 

 met with it, or of which detailed information has been obtained, muriate 

 of soda and carbonate of lime existed in the soil, and the natron was 

 found on ihe surface of the moist earth or mud. Near Gundycottah on 

 the banks of the Pennar, common salt is interstratified with the upper 

 schistose strata of the argillaceous limestone on which the sandstone 

 rests ; aud on the surface of the neighbouring soil, natron, contaminated 

 with much muriate of soda, is collected and used in washing and in glass- 

 making. The salt associated with the limestone, and the water of a 

 saline well dag through it, having the specific gravity of 1002'5, contain 



• Journal of the Rojal lastitutiou, vol. vii., p. 294. 



