The plain or basin, on the borders of which the famous bed of nitrate of soda lies, is situated at the distance of about thirty miles from the sea, being separated 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 by Mr.Belford Wilson, the Consul-General at Lima) of 420 miles.
In a well near the works, thirty-six yards in depth, sand, earth, and a little gravel were found: in another well, near Almonte, fifty yards deep, the whole consisted, according to Mr.Blake, of clay, including a layer of sand two feet thick, which rested on fine gravel, and this on coarse gravel, with large rounded fragments of rock.(See an admirable paper "Geological and Miscellaneous Notices of Tarapaca" in "Silliman's American Journal" volume 44 page 1.) In many parts of this now utterly desert plain, rushes and large prostrate trees in a hardened state, apparently Mimosas, are found buried, at a depth from three to six feet; according to Mr.
Blake, they have all fallen to the south-west.The bed of nitrate of soda is said to extend for forty to fifty leagues along the western margin of the plain, but is not found in its central parts: it is from two to three feet in thickness, and is so hard that it is generally blasted with gunpowder; it slopes gently upwards from the edge of the plain to between ten and thirty feet above its level.It rests on sand in which, it is said, vegetable remains and broken shells have been found; shells have also been found, according to Mr.Blake, both on and in the nitrate of soda.It is covered by a superficial mass of sand, containing nodules of common salt, and, as I was assured by a miner, much soft gypseous matter, precisely like that in the superficial crust already described: certainly this crust, with its characteristic concretions of anhydrite, comes close down to the edge of the plain.
The nitrate of soda varies in purity in different parts, and often contains nodules of common salt.According to Mr.Blake, the proportion of nitrate of soda varies from 20 to 75 per cent.An analysis by Mr.A.Hayes, of an average specimen, gave:--Nitrate of Soda....64.98
Sulphate of Soda....3.00
Chloride of Soda...28.69
Iodic Salts.........0.63
Shells and Marl.....2.60
99.90
The "mother-water" at some of the refineries is very rich in iodic salts, and is supposed to contain much muriate of lime.("Literary Gazette" 1841page 475.) In an unrefined specimen brought home by myself, Mr.T.Reeks has ascertained that the muriate of lime is very abundant.With respect to the origin of this saline mass, from the manner in which the gently inclined, compact bed follows for so many miles the sinuous margin of the plain, there can be no doubt that it was deposited from a sheet of water:
from the fragments of embedded shells, from the abundant iodic salts, from the superficial saliferous crust occurring at a higher level and being probably of marine origin, and from the plain resembling in form those of Chile and that of Uspallata, there can be little doubt that this sheet of water was, at least originally, connected with the sea.(From an official document, shown me by Mr.Belford Wilson, it appears that the first export of nitrate of soda to Europe was in July 1830, on French account, in a British ship:--In year, the entire export was in Quintals.
1830............................17,300
1831............................40,885
1832............................51,400
1833............................91,335
1834...........................149,538
The Spanish quintal nearly equals 100 English pounds.)THIN, SUPERFICIAL, SALINE INCRUSTATIONS.
These saline incrustations are common in many parts of America: Humboldt met with them on the tableland of Mexico, and the Jesuit Falkner and other authors state that they occur at intervals over the vast plains extending from the mouth of the Plata to Rioja and Catamarca.(Azara "Travels" volume 1 page 55, considers that the Parana is the eastern boundary of the saliferous region; but I heard of "salitrales" in the Province of Entre Rios.) Hence it is that during droughts, most of the streams in the Pampas are saline.I nowhere met with these incrustations so abundantly as near Bahia Blanca: square miles of the mud-flats, which near that place are raised only a few feet above the sea, just enough to protect them from being overflowed, appear, after dry weather, whiter than the ground after the thickest hoar-frost.After rain the salts disappear, and every puddle of water becomes highly saline; as the surface dries, the capillary action draws the moisture up pieces of broken earth, dead sticks, and tufts of grass, where the salt effloresces.The incrustation, where thickest, does not exceed a quarter of an inch.M.Parchappe has analysed it (M.d'Orbigny "Voyage" etc.Part.Hist.tome 1 page 664.); and finds that the specimens collected at the extreme head of the low plain, near the River Manuello, consist of 93 per cent of sulphate of soda, and 7 of common salt; whilst the specimens taken close to the coast contain only 63 per cent of the sulphate, and 37 of the muriate of soda.This remarkable fact, together with our knowledge that the whole of this low muddy plain has been covered by the sea within the recent period, must lead to the suspicion that the common salt, by some unknown process, becomes in time changed into the sulphate.Friable, calcareous matter is here abundant, and the case of the apparent double decomposition of the shells and salt on San Lorenzo, should not be forgotten.
