LAWSON "I 



r.\ i \c heJ 



The Berkeley Hills. 



individuals showing the crystallization of the two minerals to havq 

 been simultaneous. The augite is colorless to pale green or 

 yellowish green and is not appreciably pleochroic. It exhibits 

 characteristic cleavages, has a high refractive index, and strong 

 double refraction. The maximum extinction measured from the 

 chief axis in clinopinacoidal sections was 44 degrees; the average of 

 seven readings was 41 degrees. Twinning is very common accord- 

 ing to the augite law, the twinning plane being the orthopinacoidj 

 The twins consist of two individuals, separated generally by ari 

 irregular surface, or of many broad and narrow lamella; irregularly 

 alternating, thinning out or ending abruptly in the middle of the 

 crystal. In one case a twin was observed composed of many 

 lamellae, in which the twinning plane appeared to be the basal 

 plane. Several intergrowths of augite and hypersthene were 

 observed, the augite always inclosing the hypersthene as a periph- 

 eral zone. The two minerals have their chief axes in common, 

 and are separated by an uneven but perfectly distinct line. Inclu- 

 sions of glass in rounded blebs and magnetite in grains and crystals 

 are common, and needles of apatite were noted. Decomposition 

 of augite to serpentine is not rare in these rocks. The serpentine 

 develops about points in the center or on the periphery of the 

 augite crystal, forming sharply-defined areas from which strings 

 and bands of the serpentine extend into neighboring cleavage 

 cracks. It is a finely-felted or fibrous form of the serpentine, in 

 which, when replacement is complete, no definite remains of the 

 structure of the augite can be detected. One crystal of augite was 

 observed to be partly replaced by calcite. Chlorite is rarely 

 present in scaly aggregates, mingled with serpentine in decomposed 

 areas. It appears to be an exceptional product of augite alteration. 

 The augite of the ground-mass is invariably in the form of small, 

 rounded grains, which are identical in color and optical properties 

 with the augite of the phenocrysts. These grains fill the interstices 

 between the feldspar microlites, which they nearly equal in bulk. 

 The occurrence of augite as inclusions in the feldspar phenocrysts 

 has been described. 



Hypersthene is a constituent of one phase of the andesites, 

 but is present in small amount. It forms imperfect phenocrysts of 



