1922] Hudson: Geology of the Cuyamaca Region of California 217 



alteration was effected by meteoric waters and that the place favorable for the 

 growth of chlorite was in the sulphide ore not far beneath the oxidized zone. 



Actinolite. — The partially oxidized ore contains a considerable amount of a 

 fibrous, green amphibole. Its optical properties are: eAf = 19°, optical sign, 

 negative, a = 1.62 + , /3 = 1.630, 7 = 1.647. Colorless in thin section. This is 

 probably actinolite, though its indices of refraction are higher than those listed 

 in the tables. It is closely associated with the chlorite, flakes of the latter 

 mineral being inclosed in the amphibole. In the completely oxidized ore the 

 actinolite is white and dull, probably the result of leaching. It is thought that 

 the actinolite is a secondary mineral formed by the same agencies as was the 

 chlorite, since it increases in amount as the degree of oxidation increases, and is 

 known to form at the expense of brown hornblende. (See pi. 10, fig. 3.) 



Calcite. — White carbonate, identified as calcite, occurs as minute veinlets 

 traversing both sulphides and silicates. The relation of the calcite to the horn- 

 blende in some specimens suggests that it may replace that mineral (see fig. 18) 

 and certain masses of granular calcite are interpreted as complete replacements 

 of amphiboles and augite by the carbonate. 



Interrelations of Minerals 



If the calcite masses which show six and eight sided sections are 

 correctly interpreted as pseudomorphs after augite, it follows that the 

 augite possessed idiomorphic outlines. The compact green hornhlende 

 generally occurs in stout prisms with rounded terminations. It ap- 

 proaches closer to euhedral form than does the amphibole of the norites 

 and gabbros. The brown hornblende is found sometimes in even more 

 perfect crystals. In general it may be said that the primary silicates 

 usually have several of their bounding planes determined by crystal- 

 lographic faces, the other bounding planes being smooth curves. There 

 is little tendency for the hornblendes to be penetrated along their 

 cleavages by the sulphides. 



The sulphides make up from 75 to 90 per cent of the ore. As 

 pyrrhotite is the prevalent sulphide, it may be said that this 

 mineral determines the texture of the ore. It occurs in grains rang- 

 ing from a few millimeters to, several centimeters in diameter. In 

 either the polished surface or the hand specimen the grains may be 

 distinguished one from the other by the diverse orientation of the 

 basal partings in the different grains. The boundaries are never 

 crystallographic planes, but are made up of smooth curves, much like 

 the boundaries between the silicate minerals of the gabbros and the 

 norites. The texture of the ore further resembles that of the gabbros 

 and norites in that the grains of pyrrhotite are on the whole equant. 



Chalcopyrite is found in general as small irregular masses occu- 

 pying the space where several large grains of pyrrhotite meet. Its 



