Having passed the distance of one thousand yards, the glen again becomes narrow, until it joins with the ravine of the river of Yura, which flows from the north-east to south-west. From this place it becomes deeper and narrower, and immediately follows a westerly direction. A rivulet formed by the junction of the smaller streamlets which arise in the hill called Horqueta irrigates some land in the strath beneath; and, pursuing its course to the baths, passes them at a yard’s distance, (now ceasing to be pure water,) to unite itself with the many jets that spring up a little above the baths, as I have seen in a ditch recently opened. This rivulet follows the course of the glen; and its waters, being impregnated with saline ingredients, irrigate the lucern fields, and contribute to increase the supply of the carbonate of soda. It at length joins the river of Yura, which during the periodical rains is in the highest degree dangerous to ford, on account of its strong current, and the many stones it carries along its impetuous stream. In the glen of Yura, as well as in the ravine alluded to, the different sorts of earth are of volcanic and transition formation: the first extend to a hundred paces beyond the ferruginous baths; the prevailing rock is trachyte, of a light ash-colour, with pieces of white felspar half decomposed, pumice-stone, and scales of black mica.
In some detached masses are seen in globules the substance called perlita, (little pearl,) and black pieces, which, from their general appearance and concavities, look like lava. Ascending a little higher than the sulphurous baths, porphyry is met with; the principal mass of which is compact and black, its fracture conchoidal, and it contains crystals of white felspar. By decomposition it has partly become an ash-coloured rock, less hard and more asperate, in which is found the conduit which emits sulphurous vapours; and in my opinion it is an ancient crater. On the walls of this crater, sulphur is deposited in well-defined, acute, octahedral crystals; and, in some pieces in my possession, the pure sulphur exists in its massive state. These rocks cover a sandy ground, which from its coarse grain, composition, reddish colour, and the fragments of volcanic rocks which it contains, appears to be a sandy conglomerate; it is sufficiently consistent to admit of being cut, and to serve for architectural purposes: there are certain places, as in the Calera, and near the baths, where it is many yards thick: this earth reposes on the transition series.
The earths, or mineral substances last mentioned, occupy all the parts to the north and west: they are composed of gres, (a stone abounding in sand,) semi-compact in some layers, and in others it has a fine grain; its colour is a dirty white, inclining to green, and it passes to a lightish black when it is near to the layers of the black “esquito hojoso,” (foliated schist,) with which it alternates: it contains small spangles of mica. The natives extract laminæ of this gres of more than a yard in length, and of a quarter in breadth, which serve to line the upper part of the boilers wherein soap is made. The black “esquito” also divides itself into laminæ of good size; but, for the most part, it breaks very easily, forming small pieces which fall down to the bottom of the glen. A heavy substance of dark colour, which separates in large pieces, and effervesces when brought into contact with the acids, is found in the “esquito,” and near to the layers of gres: it appears to be a carbonate of iron, (carbonato de hierro litoideo,) like that which is met with in the coal-mines of England and France. In the “esquito” I have observed impressions of plants, vestiges of coal, crystals, and small plates of gypsum.
The transition formation extends to the north and to the west, at great distances. I am also assured that coal is met with near to the village of Yura. Over the horizontal bed of gres, the direction of which is from east to west, with an inclination northward, may be observed the white, compact, fibrous gypsum of Synchita, distant six leagues from the glen, which, as I conceive, belongs to the gypsum of the vale of Vitor.[64]
On the south side of the river Yura, in the locality called Calera, is found in layers or coats of considerable thickness, but of little extent, a limestone of a cellular and porous structure, composed in a great degree of very small and delicate tubes: their colour is a dirty white, and by all their signs and characters they appear to owe their origin to infiltration. From this stone is made the lime which is consumed in Arequipa and its neighbourhood.
ANALYSIS OF THE MINERAL WATERS OF YURA.
Mineral waters are distinguishable from common water by their taste, particular smell, colour, temperature more or less raised, and by their not being applicable to domestic purposes. They are found in different parts of the world in springs and wells: sometimes they are of the same temperature with the soil through which they pass, and at other times their temperature rises to the boiling point of water,—and then they are called thermal waters. In the countries where these waters appear, they had attracted the attention of the inhabitants since very remote times, and were medicinally employed internally as well as externally; but, since their component parts were but imperfectly known, they were often applied injudiciously, and they did not always obtain the reputation they merited, for their effects were sometimes contrary to those which the physicians desired to produce.
At the close of the seventeenth century chemists began to discover the substances to which mineral waters owed their peculiar properties; and, since this happy era, they have made such rapid progress in science, that in the present day we are acquainted with many of these substances. This knowledge we owe to simple and more exact methods of analysis. Nature appears to have favoured in an especial manner the environs of Arequipa with thermal springs to cure those maladies to which its inhabitants are subject. This is, however, no more than might be expected, considering the variety of medicinal ingredients with which the waters become impregnated as they slowly percolate through beds of lava, or issue from the deep recesses of burning mountains.