SMITHSONIAN EXPLORATIONS, I93I 



39 



oped body of sulfides of lead, zinc, and silver near one edge of the 

 district. 



The city of Queretaro, capital of the state of the same name, lies 

 among green and fertile fields and is a place of considerable histori- 

 cal interest, having been the scene of many incidents of Mexican his- 

 tory. To the south of the city are low hills capped by old flows of 



Fig. 34. — A cottage at San Pedro, Mexico. 



rhyolitic lavas that contain, in numerous places, the opal mines so 

 famous for the richness and variety of their products. The opals are 

 found in the steam cavities of these old lavas over a wide area but 

 active mining is carried on only in a desultory manner by the natives. 

 The stones are brought to Queretaro where they are cut upon common 

 grindstones and polished on wooden wheels. Clear opals with little 

 fire are abundant and cheap ; the finest stones are much rarer, more 

 costly and are of great beauty. 



