Chap. L] 



ECONOMIC GEOLOGY. 



215 



Common Salt. 



Salt and Soda. Common salt effloresces from the soil and superficial 

 rocks over a considerable area to the North of Lal- 

 goody, in the Trichinopoly district ; and is collected 

 by the poorest of the village people for household use. It is very impure, 

 containing apparently a considerable admixture of chloride of calcium, 

 which gives it a disagreeable bitter taste, and which the village people 

 have not the art of separating. It is chiefly collected from the beds of 

 nullahs, where, after a drought of a few weeks, it covers the sand with a 

 thin efflorescence. The surface sand is scraped together by the villagers, 

 lixiviated in some such arrangement as that shown in the annexed 

 sketch (Fig. 24), and the solution thus obtained, evaporated to dryness in 

 Fig. 24. Native Method of lixiviating Salt-earth. Trichinopoly. 



Soda. 



the sun, on flat stones, round which a rim of clay has been made to retain 

 the solution. 



Soda is of wide spread occurrence in the South Arcot and Trichino- 

 poly districts, chiefly on the gneiss, and on the 

 alluvium, more rarely * on the Cretaceous rocks. 

 It occurs in a whitish soil, known as "Soud" or "Dhobee's earth," usually 

 occurring in marshy places, and very treacherous to the unwary pedes- 

 trian. A large extent of it occurs between Trichinopoly and Tanjore, 

 to the South of the Cauvery ; and the alluvial plain to the North of the 

 Vellaur, in the neighbourhood of Chetia-tope and Bhonagiri, is formed in 



* Never derived from the Cretaceous Rocks, but from the decomposition of the many 

 varieties of hornblendic and felspathic gneiss. T. Oldham. 



