40 WYNNE : TRANS-INDUS SALT REGION, KOHAT DISTRICT. 



salt, being only between J and 1 per cent., but the point of saturation 

 with sulphate of lime being so much more quickly reached, gypsum 

 might become widely spread, while the water would not part with the 

 salt till continued evaporation had caused saturation thereby and greatly 

 contracted its area, (in the case of sea- water the volume of the salt only 

 representing j^th of that of the water by which it would be deposited), 

 the thick accumulations of this mineral indicating deeper pools left in 

 the basin as it dried up. Thus the gypsum might occupy a much larger 

 original area than the salt, and this area rising round the saline pools 

 might contribute largely to the formation of a gypseous deposit capable of 

 being mechanically removed to overlie the salt in the manner now seen. 



Dr. Warth has suggested that deep brine pools such as these might 

 be overflowed by more slightly saline or non-saline water bearing gypsum 

 in solution or clay in suspension, which from its less specific gravity 

 would not mix with the heavy brine below until it had reached the same 

 point of saturation, and that in this way layers of clay or of gypsum 

 could be deposited upon, or alternating with, bands of salt, the crystal- 

 lization of which would be permanently arrested by larger accessions of 

 fresher water. 



Even supposing the formation of rock-salt possible as above sup- 

 posed, the very magnitude of the enormous accumulations of excep- 

 tionally pure salt-rock in this Trans-Indus district still involves much 

 obscurity as to their source, partly perhaps from the slightness of pre- 

 sent knowledge as to what is taking place in the depths of many exist- 

 ing seas, whose waters must evaporate and be replenished by rain, and 

 also by rivers bearing mineral matter in solution from the land ; while 

 in some oceans at least the currents are shown by recent research to be 

 unfavourable to the separation of saturated brine or the formation of 

 rock-salt."* 



* Dr. Carpenter's lectures on ocean temperatures and oceanic circulation — Prog., 

 Roy. Soc, 1872, Vol. XX, 535, &c, on the ' Shearwater' Scientific Researches— to Royal 

 Institution, 1874, March 20. ' The temperature of the Atlantic'. 



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