This subject may be here conveniently treated of: I will begin with the most interesting case, namely, the superficial saline beds near Iquique in Peru.The porphyritic mountains on the coast rise abruptly to a height of between one thousand nine hundred and three thousand feet: between their summits and an inland plain, on which the celebrated deposit of nitrate of soda lies, there is a high undulatory district, covered by a remarkable superficial saliferous crust, chiefly composed of common salt, either in white, hard, opaque nodules, or mingled with sand, in this latter case forming a compact sandstone.This saliferous superficial crust extends from the edge of the coast-escarpment, over the whole face of the country; but never attains, as I am assured by Mr.Bollaert (long resident here) any great thickness.Although a very slight shower falls only at intervals of many years, yet small funnel-shaped cavities show that the salt has been in some parts dissolved.(It is singular how slowly, according to the observations of M.Cordier on the salt-mountain of Cardona in Spain "Ann.
des Mines, Translation of Geolog.Mem." by De la Beche page 60, salt is dissolved, where the amount of rain is supposed to be as much as 31.4 of an inch in the year.It is calculated that only five feet in thickness is dissolved in the course of a century.) In several places I saw large patches of sand, quite moist, owing to the quantity of muriate of lime (as ascertained by Mr.T.Reeks) contained in them.From the compact salt-cemented sand being either red, purplish, or yellow, according to the colour of the rocky strata on which it rested, I imagined that this substance had probably been derived through common alluvial action from the layers of salt which occur interstratified in the surrounding mountains ("Journal of Researches" page 444 first edition.): but from the interesting details given by M.d'Orbigny, and from finding on a fresh examination of this agglomerated sand, that it is not irregularly cemented, but consists of thin layers of sand of different tints of colour, alternating with excessively fine parallel layers of salt, I conclude that it is not of alluvial origin.M.d'Orbigny observed analogous saline beds extending from Cobija for five degrees of latitude northward, and at heights varying from six hundred to nine hundred feet ("Voyage" etc.page 102.M.d'Orbigny found this deposit intersected, in many places, by deep ravines, in which there was no salt.Streams must once, though historically unknown, have flowed in them; and M.d'Orbigny argues from the presence of undissolved salt over the whole surrounding country, that the streams must have arisen from rain or snow having fallen, not in the adjoining country, but on the now arid Cordillera.I may remark, that from having observed ruins of Indian buildings in absolutely sterile parts of the Chilian Cordillera ("Journal" 2nd edition page 357), I am led to believe that the climate, at a time when Indian man inhabited this part of the continent, was in some slight degree more humid than it is at present.): from finding recent sea-shells strewed on these saliferous beds, and under them, great well-rounded blocks, exactly like those on the existing beach, he believes that the salt, which is invariably superficial, has been left by the evaporation of the sea-water.This same conclusion must, I now believe, be extended to the superficial saliferous beds of Iquique, though they stand about three thousand feet above the level of the sea.
Associated with the salt in the superficial beds, there are numerous, thin, horizontal layers of impure, dirty-white, friable, gypseous and calcareous tuffs.The gypseous beds are very remarkable, from abounding with, so as sometimes to be almost composed of, irregular concretions, from the size of an egg to that of a man's head, of very hard, compact, heavy gypsum, in the form of anhydrite.This gypsum contains some foreign particles of stone; it is stained, judging from its action with borax, with iron, and it exhales a strong aluminous odour.The surfaces of the concretions are 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 process 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 fragments of rock from the larger masses.("Volcanic Islands" etc.page 87.) I believe the superficial gypseous deposit is widely extended: I received 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 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.("Voyage Geolog." etc.page 95.) These widely extended, superficial, beds of salt and gypsum, appear to me an interesting geological phenomenon, which could be presented only under a very dry climate.
