138 



The ore is copperglance, tile ore or impure red oxyd, and some 

 copper pyrites. Accompanying these deposits, and also where no 

 copper ore is visible, the line of contact is occupied by masses of 

 magnetic iron. Where the same limestone comes in contact with 

 diorite, the former contains large crystals of magnetic iron and 

 spinel, 



Planchas de ba Plata. — In Sonora, just south of the line, and 

 near the meridian of Tubac, are the Plai^chas de la Plata mines, 

 still celebrated throughout the Republic. According to the best 

 Mexican and Jesuit authorities, large masses of native silver were 

 discovered there in 1769. Pieces of great size were obtained, 

 (one is said to have weighed 3,600 pounds) and the workings were 

 being prosecuted with vigor and success, when the Spanish Govern- 

 ment declared the deposit to be a oriadero, and as such to belong 

 to the Crown. 



The place was therefore abandoned, and every attempt made at 

 regular working since the revolution has been frustrated by the 

 Apaches. 



The most singular feature connected with the discovery is, that 

 no vein, from which these masses could have come, was found. 

 The deposit seems to have been a regular placer. The silver oc- 

 curred in pieces of every size down to small grains. Several rich 

 veins were opened in the neighboring mountains, but were also 

 abandoned from absence of protection. The only specimens that I 

 have seen from this locahty were apparently a partially decomposed 

 quartziferous porphyry, from the wall rock of the Mina Colorada, 

 and were impregnated with grains of silverglance. 



General Conclusions. — Before the working of mines in Arizona 

 can become regular and profitable, many changes will be necessary. 

 The Apaches must either be exterminated or reduced to complete 

 submission, and this can only be accomplished by a long series of 

 campaigns. A port is also necessary, without which all supplies 

 and machinery have to be transported over deserts from the Gulf 

 of Mexico or the Colorado river. Guaymas, three hundred and 

 fifty miles, and Port Lobos, one hundred and fifty miles from Tubac, 

 are the natural entrances to the country, and so long as these re- 

 main in the hands of a treacherous and capricious government, no 

 enterprise can flourish either in Arizona or Sonora. Further, the 

 present unnatural boundary line will always be a source of trouble, 

 affording a shelter to the robbers and assassins of both countries. 



The substitution of white for peon labor, would probably be a 

 failure, owing to the debilitating influence which the climate exerts 

 on northerners. The Mexican labor is good when properly super- 



