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292 GEMS AND PRECIOUS STONES IN THE
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same
rock. Veins of porphyry are met with in regular banks, which in many
cases preserve the same constant direction as on the hill of Ceja de
Leon, southeast to northwest. The porphyry is of a grayish-red color,
although in some parts it is lighter colored, changing into a
reddish-white, even on the surface where it is altered. The aspect of
the porphyry indicates generally the class of opals it contains. Where
the rock is brick-red in color, compact and hard, the varieties with a
fiery-red color abound, also the color combined with red, formed from
different changeable colors, or rather a mixture of colors. However,
where the porphyry is lighter colored and mottled, noble opals are
found more abundantly, notably in the mines situated on the hill of
Peineta." These mines are remarkable for their richness and the variety
of their product. In a single piece of rock, from the mine of
Sim-patica, Dr. Barcena found noble opal, fire opal, harlequin opal,
and the lechosos opal. One of the mines at which the greatest amount of
work has been done is the Jurado. Here an excavation fully 150 feet
deep, 100 feet wide, and several hundred feet long has been made, and
at the depth of 150 feet the porphyritic rock contains an abundance of
hydrated silica and common opal. A deposit of opal was discovered in
1851 on the brow of the hill of the Navajas, at a place called
Tepezala, by Juan Orozca and Juan Hill, two pupils of the Mexican
School of Mines. These were fire-opals, in a conglomerate consisting
for the greater part of trachytic porphyry. In the borough of
Tepoentitlan, in the district of San Nicholas del Ora, and near
Huitzuco, both in the State of Guerrero, fire-opals are also found,
either light topaz-yellow in color, with green, red, and yellow
reflections, or white changing into reflections. Common and fire-opals
have also been observed north of San Luis Potosi, on the ridge of Mount
Mezquitic, and an opal district of considerable extent has been
observed on the Ciervo estate, three leagues south of Cadereita, and
fourteen leagues from Esperanza. A quantity of semi-opals, cacholong,
and hyalite has been obtained here, but no true opal. In this locality,
the hills are of porphyry, yet the opal-bearing-rock is readily
detected by the appearance of the soil which covers it. In the district
of Amealco, in the State of Quere-taro, opals were observed on the
Batan, Galindo, and Lallare
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