“Extensive beds of common opal and semi-opal are seen along a belt extending through the central part of the department of Gracias; but these varieties, though very beautiful and possessing high interest to the mineralogist, are without commercial value from the ease with which artificial products may be made which precisely resemble them. The localities worthy of exploration are those in which the opal forms veins (not beds) in compact but brittle trachyte of a dark color. The veins, as will be seen, are not confined to such rock, but seem to have their origin in it, and are probably not found except in connection with it. The best-known mines of precious opal are in the department of Gracias; several localities have yielded valuable gems, but they are all remote from the line of road. Some are in the vicinity of the town of Gracias, others near Intibucat; but the most important are at Erandique. The working is now carried on in a very small way; but the locality is extensive, and in my opinion mining on a large scale would be attended with profit. The country near by abounds with beds of common opal, as in many other places, but the gems occur in somewhat irregular veins running in a northeast and southwest direction, and with a nearly perpendicular dip. The veins are not continuous, but branch off and disappear at short intervals; neither are the contents of uniform quality, but the valuable parts are usually in belts in the vein, and limited on each side by portions of ordinary opal without play of colors. These lines of light are sometimes numerous and narrow, alternating with the common opal forming a very beautiful gem. Many again, even of large size, are uniform in structure, and exhibit a play of colors as brilliant as the finest opals from Hungary. The hill in which they are found is about two hundred and fifty feet high, and two or three miles in length, and for a width of half a mile for its whole length opals have been found wherever excavations have been made. The rock in which they occur is a hard, brittle trachyte of a vitreous lustre, and splintering into acute fragments when struck; a bed several feet in thickness overlying this rock is of a gray color and soft consistence, and also contains opal veins; it is probably a trachyte changed by atmospheric action.