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slate, which is also in part covered by a slight deposit of the usual 

 quaternary. The hills bounding the valley on the north and south 

 ax'e of (^uartziferous porphyry. This is a fine-grained rock, with 

 pink crystals of orthoclase and quartz crystallized in double 

 pyramids. 



The northern hne of contact between the clay-slate and porphyry 

 is marked by a bold vein of quartz running east and west. In this 

 are several openings, made previous to the Apache war. The ore 

 which I observed was galena, and its altered products disseminated 

 in quartz. It is said to contain gold. Several quartz veins travers- 

 ing the porphyry have been worked for gold, as have also the beds 

 of the arroyos in the neighborhood. 



Arivaca has too little wood for extensive operations. When the 

 Heintzelman mine is again worked, the reduction should be eifected 

 at Tubac, where the erection of large works would be an incentive 

 to the opening of many of the mines in that neighborhood. 



Santa Rita. — The mines of the Santa Rita are situated in and 

 around a beautiful valley, about ten miles east of Tubac, and among 

 the foot-hills of the Santa Rita mountains. The valley and the hills 

 to the north are of a metamorphic quartziferous porphyry, while the 

 hills to the east consist of a feldspathic rock. It is in these two 

 formations that the veins occur. 



The hills to the south are formed in part by the porjihyry con- 

 glomerates already mentioned, and in part by a remarkable feld- 

 spathic porphyry. This last rock has a compact light gray ground, 

 bearing numerous crystals of a white triclinic feldspar and small 

 prisms of hornblende, but entirely free from quartz. It is appar- 

 ently older than the conglomerates. In it no veins have been dis- 

 covered. 



The veins in the feldspathic rock are very numerous, and have with 

 few exceptions a nearly east and west course. Their dip is nearly 

 vertical, and they vary from ten to twenty-five inches in thickness. 

 The gangue is almost entirely quartz, and the ore generally ar- 

 gentiferous gray copper and galena. When this last mineral is 

 unaccompanied by the tetrahedrite, its yield is rarely over 0,1 per 

 cent, of silver, but when occurring in proximity to that mineral it 

 contains often from 0.5 to 0.75 per cent. 



The gray copper ores vary from light steel gray to tarnished 

 black, and contain from one to over two per cent, of silver. This 

 mineral, when associated with galena in decomposing, is replaced 

 by a porous vitreous substance of yellowish green color, and con- 

 sisting principally of antimonate of lead, containing from one to two 

 per cent, of silver. The " crystal vein''^ is of a massive ore of ga- 

 lena, with about twenty per cent, of zinc-blende and copper pyrites. 



