304 Nitrate of Soda. paet n. 



marked by sharp, radiating, or bifurcating ridges, as if 

 they had been (but not really) corroded : internally 

 they are penetrated by branching veins (like those of 

 calcareous spar in the septaria of the London clay) of 

 pure white anhydrite. These veins might naturally 

 have been thought to have been formed by subsequent 

 infiltration, had not each little embedded fragment of 

 rock been likewise edged in a very remarkable manner 

 by a narrow border of the same white anhydrite : this 

 shows that the veins must have been formed by a pro- 

 cess of segregation, and not of infiltration. Some of 

 the little included and cracked fragments of foreign 

 rock are penetrated by the anhydrite, and portions have 

 evidently been thus mechanically displaced : at St. 

 Helena, I observed that calcareous matter, deposited by 

 rain-water, also had the power to separate small frag- 

 ments of rock from the larger masses. I believe the 

 superficial gypseous deposit is widely extended : I re- 

 ceived specimens of it from Pisagua, forty miles north 

 of Iquique, and likewise from Arica, where it coats a 

 layer of pure salt. M. d'Orbigny l found at Cobija a 

 bed of clay, lying above a mass of upraised recent shells, 

 which was saturated with sulphate of soda, and included 

 thin layers of fibrous gypsum. These widely extended, 

 superficial, beds of salt and gypsum, appear to me an 

 interesting geological phenomenon, which could be pre- 

 sented only under a very dry climate. 



The plain or basin, on the borders of which the 

 famous bed of nitrate of soda lies, is situated at a 

 distance of about thirty miles from the sea, being sepa- 

 rated from it by the saliferous district just described. 

 It stands at a height of 3,300 feet ; its surface is level, 

 and some leagues in width ; it extends forty miles 

 northward, and has a total length (as I was informed 

 1 • Voyage Geolog.' &c. p. 95. 



