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]