There is in these vast plains through which the rivers pass an immense tract of land, the soil of which is saturated with fossil salt. It extends to the south of Buenos Ayres, and is about 700 miles in length by 150 in breadth. It is said that in this extent, which reaches to the Rio Vermejo, there is not a river, well or lake whose waters are not brackish. All the rivers which flow through it to the La Plata are fresh until they cross this waste, after which they become salt till they enter the great stream. Even the Pilcomayo and Vermejo, although they have a free course, have always a salt taste when the waters are low. This substance appears in the greatest abundance between Santa Fé and Cordova, and the salt quality of the soil reaches to St. Jago del Estero, where the whole ground is covered with a white incrustation even to the foot of the Cordillera. Natural saltpetre is also collected in this part of the country, after a shower the ground being whitened with it. Chaco contains many salt lakes, and to the south-west of Buenos Ayres, they are found at from 400 to 450 miles distance. To these, journeys are frequently made with carts, in order to collect the fine crystallized grains which cover their banks. The cattle of this country cannot subsist without this substance; they devour with avidity the salted clay they find in the ditches; and when this happens to fail, as is sometimes the case in Paraguay, they perish in the course of a short time. From Buenos Ayres, the great road to Potosi and Lima passes through Tucuman. In 1748, regular stages were built all the way, post-houses were erected, and relays of horses and carriages provided. The method of travelling is in covered waggons drawn by oxen or horses, in which the traveller can recline, and must necessarily exercise much patience: but the silver and gold from the mines, as well as all kinds of merchandize, are conveyed along this road on the backs of mules. Its extent from Buenos Ayres to Potosi, is 1617, or according to some accounts, 1873 miles, 400 of which are over the elevated chains of the Andes, and are impassable for the waggons; from Potosi to Lima the route continues 1215 miles more, and passes over the highest ridges of the mountains, where the traveller undergoes all sorts of danger and privations, and is exposed to the utmost extremes of heat and cold.