The geological features of the immediate neighbourhood of the Santa Fé Mine, which is situated about nine leagues nearly due east of the departmental town of Pichucalco, have been partially described by the author in a paper recently read before the Institution of Mining and Metallurgy. The key to the position, from the point of view alike of mineralogical and of economic interest, is found in a dome-shaped mass of wollastonite in a nearly pure condition, the horizontal section of which, at the lowest level where it has been sufficiently explored to determine its shape, takes the form of an irregular ellipse about 400 yards long by 160 yards wide. With the exception of the ore-bodies, which are developed on the outskirts of this mass and close to the contacts with other rocks, the whole mass is of nearly uniform composition, consisting of the mineral wollastonite in a nearly pure condition and very coarsely crystallized.