ART. 23 



DURANGO MINEEALOGY AND GEOLOGY FOSHAG 



slopes. It is uncovered by the excavation of the tolva on the western 

 slope and can be conveniently studied there. About the ore bodies 

 it is often altered to a pyroxenitic rock or strongly hydrated to a soft 

 tlayey mass that still retains its original structure. 



Above the latite lies rhyolite tuff, fine grained or made up of frag- 

 ments that do not greatly exceed 1 inch in size. This borders the hill 



on the north and east with a small patch or two on the south slope 

 below Picacho de la Cruz. This is well exposed by workings at the 

 Socavon de la Cueva. 



The rhj'olite is confined to the lower northern slopes extending to 

 an unknoAvn distance to the north, where it follows the ridge of El 

 Pedrigoso. Khyolite also occurs in a small hill a short distance cast 

 of Cerro Mercado, known as Cerro del Almagre, and in another 

 small hill a short distance south of the main mass. 



