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



317 



of garnet formation might have gone on, and probably did go 

 on, after the actual solidification of the intrusive mass. 



Regarding the origin of the iron ore little can with confi- 

 dence be stated. This much is known, however, that bodies of 

 iron ore similar to those occurring in this district, and often on 

 a much larger scale, are common at the contact of intrusive rocks 

 with limestone. In some cases these ore bodies are known to 

 have originated long after the solidification of the igneous rock 

 and to have resulted from the leaching of the iron from it and 

 its precipitation at the contact with the limestone. It would, 

 therefore, again not seem necessary to regard the iron ore as 

 in any way the product of the direct reaction of the molten 

 magma upon the limestone, but an after product, due to the 

 precipitation of the iron oxide at the contact of the limestone 

 from soluble salts due to the downward leaching of the por- 

 phyry. The particular reaction which has effected the precipi- 

 tation of iron oxide by carbonate of lime under such circum- 

 stances is not known, but doubtless a chemical investigation 

 would make it clear. 



At the contact of the porphyry with the monzonite of "Weary 

 Flat the former is curiously modified locally. In some prospect 

 pits that have been opened along the wagon road between Copper 

 Flat and Pilot Knob, the monzonite is exposed in proximity to 

 the overlying porphyry. Here, the only part of the monzonite 

 that is left is the large porphyritic crystals of orthoclase, and 

 even these are charged with limonite giving them a yellow color. 

 The form and cleavage of the orthoclases are intact and the 

 faces reflect the light well. All the rest of the rock, or the entire 

 matrix in which the porphyritic orthoclases are imbedded, has 

 been altered to a reddish cellular aggregate of limonite and 

 silica. In thin sections of this altered rock the orthoclase is 

 perfectly fresh, showing no kaolinization products, although con- 

 taining much limonitic pigment. Carlsbad twinning is common, 

 and in some sections free from limonite a good biaxial figure 

 was obtained. The index of refraction is less than that of bal- 

 sam. The matrix in which these large orthoclases are imbedded 

 appears under the microscope to be composed of nothing but 

 limonite and secondary quartz in rather large grains and aggre- 



