Vol. 4] Lawson. — The Robinson Mining District. 



309 



feldspars range up to about 4 mm. in their longest dimension, 

 and the hornblendes attain a length of 6 mm., while the large 

 orthoelases are usually from 10 to 20 mm. in length, though in 

 some phases of the rock they are as much as 50 to 75 mm. long 

 and proportionately broad. In other specimens of the rock from 

 the same locality no quartz can be detected with certainty, and 

 the orthoclase and plagioclase, the latter having a maximum ex- 

 tinction angle in the zone normal to (010) of 23°, occur in about 

 equal proportions. 



In the vicinity of the copper bearing porphyry four occur- 

 rences of this monzonite porphyry have been found, all of them 

 quite limited in extent. Of these, two are found in the Ruth 

 limestone, one in the Arcturus limestone, and one in the por- 

 phyry itself, but so near its contact with underlying limestone 

 that it is quite doubtful whether it really cuts the porphyry or 

 not. They all have the same general microscopic characters 

 and, although their actual contact with the surrounding rock 

 is in no case exposed, they can only be interpreted as dykes. 

 In the northwest corner of the Spion Kop claim the monzonite 

 porphyry dyke presents a greenish gray matrix in which are 

 imbedded flesh tinted orthoelases with Carlsbad twinning, an 

 occasional crystal of yellow titanite, nests of epidote and some 

 scattered cubes of pyrite. In thin section the rock is seen to be 

 much decomposed. The ground mass is a holocrystalline, pan- 

 idiomorphic aggregate, consisting chiefly of feldspar in which 

 secondary calcite, epidote and pyrite occur very commonly. In 

 this are imbedded the large orthoelases, titanite, a little quartz 

 and a ferromagnesian mineral changed almost wholly to chlorite. 

 No lamellar twinning was observed in the feldspars, but it is 

 probably obscured by the decomposition which has affected them. 



A similar dyke is exposed in a small prospect hole near the 

 forks of the road to the west of the Ruth Mine. The rock is the 

 same as that just described except that apatite is a notable fea- 

 ture of the slides. 



Another occurrence is in the saddle of the ridge just north 

 of the Blair claim. The rock here has a compact gray matrix 

 in which are imbedded large orthoclase crystals up to 25 mm. 

 in length with Carlsbad twinning and fresh lustrous cleavages, 



