GEOLOGY OF THE SAN JOSE DISTRICT 275 



in diameter, and the grains of magnetite which are fairly abun- 

 dant in the rock are still smaller. The ground-mass is a dense 

 aggregate of pieces of quartz and kaolinized feldspars. These 

 are on the average .05 mm. across and appear to be in nearly 

 equal proportions. 



.{b) Imogenia Type. — The type here described occurs along 

 Vegonia and Bretana Creeks in the foothills of the Baril range. 



Macroscopic Description. — In the field this rock when un- 

 weathered is grayish in color and shows a rough fracture. It 

 is medium-grained and porphyritic in texture with a predomi- 

 nance of feldspar phenocrysts having bright cleavage faces as 

 against the smaller proportions of the aphanitic ground-mass. 

 Dark specks, shown by the microscope to be pyroxene, are 

 scattered through the hand specimen. 



Microscopic Characters. — The examination of this rock in thin 

 sections shows that it is made up of plagioclase, orthoclase, 

 quartz, augite, hornblende, biotite, magnetite, apatite and chlor- 

 ite. Owing to the intimate mixture of quartz and feldspar in 

 the ground-mass considerable difficulty is found in estimating 

 the relative proportions between them optically. It is probable 

 that there is an error of 5 per cent, in the subjoined table, which 

 could not be guarded against. The mineral percentages by 

 weight, as given in it, are however approximately accurate. 



Plagioclase 32 



Orthoclase 25 



Quartz 17 



Augite 16 



Biotite 2.4 



Magnetite 6.7 



Hornblende 

 Apatite 

 Titanite 

 Chlorite 



1 00.0 



The phenocrysts are of feldspar and quartz, augite, some 

 biotite and the older generation of the magnetite. The acces- 

 sory minerals make up perhaps i per cent, of the quartz-feldspar 

 ground-mass. 



