Vol. 4] 



Law <so n 



, — The Robinson Mining District. 



315 



may of course have been vents in the arch to the surface as it 

 existed at the time of the intrusion. 



Contact MetamorpJiism. — The conclusion which has been 

 reached on purely structural grounds as to the intrusive char- 

 acter of the porphyry is confirmed by evidence of contact action 

 which has been detected at various points on the periphery of 

 the mass. The southern border of the porphyry to the east of 

 the Ruth Mine is characterized by a zone of vermilion colored 

 earthy material which lies between it and the limestone. To 

 the south of the porphyry, immediately opposite the Ruth Mine, 

 the- first rock that one meets is passing from the intrusive to- 

 ward the Ruth limestone is a body of yellowish green garnet 

 rock associated with magnetic and limonitic iron ore. 



The garnet rock has frequently a miarolitic structure, show- 

 ing cavities with drusy crystals of garnet up to 2 mm. in diam- 

 eter. Occasionally there is a green copper stain. Even the most 

 massive phases of the rock appear in thin section to be made up 

 of an aggregate of fairly well formed crystals. The garnets are 

 isotropic. There are occasional small veins of quartz traversing 

 the garnet, and small areas of yellow stained quartz aggregates 

 occur. The iron ore with which this garnet rock occurs is found 

 in considerable bodies over several acres of ground but is not 

 intermixed with the garnet. In hand specimens it is feebly 

 magnetic, shows yellowish stains and has the yellow streak of 

 limonite. In thin section it is chiefly opaque, but is shot through 

 with irregular areas and veinules which are translucent to red- 

 dish yellow light. The ore is evidently a mixture of limonite 

 and magnetite. A sample analyzed by Mr. Ross contained 65.6 

 per cent. iron. 



At the contact of the porphyry with the Ely limestone on 

 the Jupiter claim, there have been developed bodies of yellowish 

 green garnet rock which, in thin section, is seen to be an aggre- 

 gate of more or less idiomorphic crystals of doubly refracting 

 garnet, with angular spaces between the crystals filled with 

 calcite. 



At the same contact on the Emma G. claim, the limestone 

 has been changed to a white marble or a mixture of calcite and 

 garnet. In places it may be called a garnet rock with a subor- 



