320 



University of California Publications. 



[Geology 



lying in a felt work. This felted matrix is quite open and porous 

 in texture. Under the microscope the white scaly mineral of 

 the matrix has the properties of kaolin and has some quartz 

 associated with it. The feldspar phenocrysts are represented 

 by an aggregate of white mica scales and quartz. The broad 

 blades prove to be white mica probably a bleached biotite. One 

 crystal of zircon was detected and a few minute crystals of brown 

 hornblende. 



On the hill to the west of Copper Flat shaft house the por- 

 phyry is yellowish in appearance and with the lens is seen to 

 be of a porous or honeycomb texture. Quartz veins 5 or 6 mm. 

 thick traverse it, also small zones of secondary quartz without 

 definite walls. The porphyritic structure is quite apparent, but 

 the feldspars are of a dull earthy appearance, as is, indeed, also 

 the matrix in which they lie. There is no effervescence with 

 acids. 



In thin section much of the field is turbid with yellow stained 

 scarcely translucent decomposition products. Between patches 

 of this kind is an aggregate of untwinned feldspar and quartz. 

 Cavities are numerous. Pseudomorphs of sharply idiomorphic 

 feldspars, in a very turbid condition, are numerous. 



On the south side of same hill as that on which the last 

 specimen was taken, the porphyry is a dull white rock with 

 drusy cavities and irregular patches of blue gray quartz. There 

 are very faint traces of a porphyritic structure. In thin section 

 the ground mass consists of untwinned feldspar and some quartz 

 in which are occasional phenocrysts of orthoclase with Carlsbad 

 twinning and numerous larger grains and areas of secondary 

 quartz. A brownish isotropic decomposition product occurs in 

 patches through the slide and there are besides these stains of 

 limonite. 



At the Copper Flat Mine the porphyry when charged with 

 sulphides becomes a copper ore. A sample of the ore from the 

 dump of the mine is a dull white rock showing occasional cleav- 

 ages of feldspar phenocrysts which is traversed to the extent of 

 half its volume with light colored quartz stringers ramifying 

 in all directions. Both the dull white rock and the quartz have 



