1886.] NATURAL SCIENCES OP PHILADELPniA. 279 



The principal mines are on the hacienda of Esperanza, where 

 the opal was discovered, by a servant, ten years ago. No mines 

 were taken up until 1870, when Di\ Jose Maria Siurob located 

 the mine of Santa Maria Iris. The fine specimens secured during 

 the next few years created so much excitement that a large 

 number of mines were located, most of which are now abandoned. 

 The district is quite extensive, having been traced over a region 

 about twenty leagues long by thirty-one leagues wide. At 

 Ciervo, fourteen leagues from Esperanza, the opal is quite 

 abundant, though none of the precious variety of good quality 

 has been found. 



The mines of Esperanza can only be reached on horseback, 

 and the ride from Qneretaro is a very hard one. San Juan del 

 Rio, said to be the nearest large town where one can get accom- 

 modations for the night, is ten leagues to the S. E. 



Leaving the Mexican Central Railroad and crossing a rich 

 alluvial plain, covered with fields of corn and the so-called cen- 

 tury plant, surrounded by fences of Gereiis giganteus, we came 

 to the foot hills. Here at once abundant evidences of volcanic 

 action were seen. Round nodules of obsidian, large masses of 

 agate, milk opal, and other siliceous products, Avere mixed with 

 the cacti on every side. The rough trail soon led into the dry 

 bed of a barranca, where porph3-ritic trachyte carrying tlie 

 common varieties of opal were quite abundant ; the trend of 

 these porphyritic banks was from S. E. to N. W. The color of 

 the rock is reddish gray. As the barranca terminated in a 

 narrow valley, on the mountains on each side were seen the red 

 dumps standing out conspicuously upon the gray surface. 



Of the mines that he visited only one, the Jurado, was being 

 Avorked. The deposits of opal-bearing trachyte are so irregular 

 that the mines are soon exliausted. 



The Jurado is an immense excavation about 150 feet deep 

 several hundred feet long, and about 100 feet wide. At the 

 bottom, the porphyritic rock seemed to be thoroughly impreo-- 

 nated with hydrated silica, even occasionally being converted 

 into common opal. 



The general appearance of the rock furnishes a very good clue 

 to the character of the opals that it may contain. Thus if the 

 rock is less red in color and close and compact in texture, fire 

 opals and related forms abound, while if the rock is deep red in 

 color or clayey and pockety, the Hungarian, harlequin, and milky 

 opals are much more abundant. 



There is no locality of which he had ever heard where such 

 an extraordinary variety of opals can be found in a sino-le 

 matrix. The same small piece of rock will show fire opal, fire 

 opal showing green and blue reflections, the Hungarian, harlequin 

 girasol, h3'alite, milk, and almost every vai-iety. The harlequins 

 showing a mosaic of brilliant minute spangles of color in a 

 milky base, vie with the broad sheets of dazzling blue, red and 



