“Analysis does not show the two sulphates to be in definite proportions in the masses, but the crystals may be a double salt, composed of one equivalent of sulphate of soda and one equivalent of sulphate of magnesia, each retaining an equivalent of water. In the masses, the closest approximation is 42 parts of sulphate of magnesia found, instead of 46 parts required.”
The communication presented embraces interesting facts. These saline deserts cover extended areas in different parts of South America, and, so far as the author has been able to learn, the saline matter differs in kind at the different points. The tendency of saline matter contained in any soil is to rise through the aid of moisture to the surface, where, the water escaping, the salt is deposited. This effect, contrary to the gravitating influence, is the most common cause of deserts, and may be exerted everywhere when the evaporation of water from a given surface becomes much greater than that surface receives in the form of rain and dew. The cultivation of saline deserts, by washing down the saline matter, exhibits the opposite action of water in restoring fertility, and it is by no means essential that the water should contain organic, matter to insure the full effect, as the soil of deserts generally contains all the organic matter of many years’ accumulation.
An interesting inquiry naturally presents itself to the traveller while crossing this peculiar desert. By what means was the salt deposited? Two theories have been advanced by gentlemen who have visited the travesia, both to account for its presence.
Mr. Bland, the North American Commissioner, who visited the Argentine Republic in 1818, thinks that these plains “may have been gently lifted just above the level of the ocean, and left with a surface so unbroken and flat as not yet to have been sufficiently purified of its salt and acid matter, either by filtration or washing.”
Sir W. Parish’s idea of the origin of the salt is different. He says, “But is it not more likely to have been washed down from the secondary strata, which form the base of the Andes, in which we know that enormous beds of salt abound, particularly in those parts of the Cordillera where the greater number of the rivers rise which run through the pampas, and which are almost all more or less impregnated with it?”
While crossing the pampas I occasionally noticed that the water of some of the streams was brackish, but as we approached the Andes the water of the rivers was pure, and free from salt. The San Juan and Mendoza Rivers, both of which may be called great torrents, bring down alluvial mud in their currents; but I never was able to detect any saline properties either in the mud or water. The natives, however, have assured me that there are many salt mines in the Andes.
CHAPTER XII.
ON THE TRAVESIA.
On the 28th of April our caravan crossed the River Desaguadero, and upon the western bank the peons killed an ox, and we ate for the first time since the morning of the previous day. At noon we reached the limits of artificial irrigation, which is carried on extensively in the neighborhood of Mendoza. Along the road ran a shallow ditch, four feet wide, and containing about two inches of water, which, when the canal is full, fertilizes the soil in the vicinity.
Beyond the Desaguadero, forty leagues from Mendoza, lay the hamlet of La Paz, upon the outskirts of which we encamped for the night. Very different was this hamlet from the others we had passed, which looked old and squalid, the houses seeming ready to crumble in pieces, and little vegetation, save in San Luis, was to be seen. Here everything looked neat, and a degree of comfort prevailed that was refreshing to the eye of the traveller who had just crossed a dreary country. This comfortable and fresh appearance was the result of irrigation, for very little rain falls on this great travesia, which covers many thousand miles of territory in the provinces of San Luis, Mendoza, and San Juan; and wherever the water of rivers can be turned from their natural course to fertilize the broad waste lands, there little spots of verdure appear, and the labor of the farmer is crowned with success.
The whole township of La Paz was divided into square pastures, around which ran a wide canal. Along the borders of these grew tall poplars, that served to fence in and protect the herds of cattle that had been brought from San Luis to fatten on alfalfa, a species of clover. Our patron was so parsimonious that he refused to purchase good pasturage for the cattle, which were growing weaker each day on miserable fare, but ordered the capataz to drive them to a piece of waste land, upon which grew a scanty supply of dry grass.