FRANCISCAN GROUP 



39 



group throughout much of its wide extent, although 

 in a few areas, such as one a few rniles west of Healds- 

 burg, in Sonoma County, the metamorphic rocks are 

 more abundant and better developed. 



HORNBLENDE ROCKS 



Hornblende schists, and similar but less schistose 

 rocks called amphibolites, are among the most com- 

 mon metamorphic rocks in the district, but because 

 they are not so striking as the beautiful blue glauco- 

 phane rocks they are more easily overlooked. Most 

 of them crop out on Mine Hill or in the vicinity of 

 the Guadalupe mine, but small bodies also occur along 

 the margins of serpentine masses extending southeast 

 of Mine Hill and in the central part of the Santa 

 Teresa Hills. (See pi. 1 and fig. 28.) Their field re- 

 lations are commonly obscure because these rocks 

 weather readily to form a characteristic red soil, 

 through which the more resistant kinds protrude as 

 low knobs. Where roadcuts or adits offer good ex- 

 posures, one may observe that the distribution of the 

 hornblende rocks is spotty, and in areas of knobby 

 outcrops not all of the intervening rock is metamor- 

 phic. Consequently, most of the larger areas mapped 

 as hornblende rock actually contain a "matrix mate- 

 rial" of greenstone tuff as abundant as the hornblende 

 rocks, although in some areas, such as the one about 

 half a mile west of the Guadalupe mine, the amount 

 of amphibolite is considerably greater than the amount 

 of admixed greenstone. 



Megascopic features 



Although typically the hornblende rock is entirely 

 dark green or almost black, some that contains albite 

 is flecked with light areas elongated parallel to the 

 schistosity. The diameters of the constituent mineral 

 grains are variable, but in most specimens the average 

 grain size is a little more than 1 mm. Surfaces broken 

 parallel to the poorly developed schistosity are rough 

 and irregular, but owing to the amphibole cleavage 

 planes, they exhibit a sparkle by which the hornblende 

 rocks may be distinguished from the coarser varieties 

 of greenstone. Less typical, but more easily recog- 

 nized, are foliated hornblende gneisses with alternat- 

 ing bands rich in hornblende, albite, or epidote, and a 

 few varieties contain porphyroblasts of pink garnet, 

 which form an obvious flaser texture. Irregular veins 

 of albite, quartz, epidote, or chlorite cut these rocks 

 in many places. 



Microscopic, features 



The principal minerals found in thin sections of the 

 hornblende rocks are hornblende, epidote, chlorite, al- 

 bite, and garnet; minerals found in smaller quantity 

 include common actinolite, blue-green actinolite, zois- 

 ite, clinozoisite, zircon, apatite, sphene, leucoxene, 

 pyrite, and magnetite. The hornblende forms stubby 

 subhedral crystals that are generally oriented only to 

 the extent of having their c axes nearly in the plane 

 of schistosity. It is common hornblende and shows 

 little variation among the specimens examined; most 

 of it is moderately deep colored and pleochroic with 

 X pale yellowish green, Y olive green, and Z faintly 

 bluish green ; birefringence ranges from 0.020 to 0.028 ; 

 the extinction angle between the slow ray and the c 

 axis is about 20 ; 2F is large, and the optic sign is 

 negative. The other amphiboles occur in minor 

 amounts as needles in albite or as thin overgrowths 

 on the hornblende. (See fig. 29.) The plagioclase 

 in all specimens examined is albite; it occurs mainly 

 as intricately sutured interlocking crystals in irregular 

 veinlets or as interstitial patches. The minerals of 

 the epidote group generally form subhedral crystals 

 which tend to be concentrated in layers. Garnet forms 

 fractured porphyroblasts several millimeters in diam- 

 eter, and in many specimens it is partly converted to 



FIGURE 29. Photomicrograph of hornblende-albite gneiss. Hornblende 

 (H), albite (A), epidote (E), and blue-green sodic amphibole (sa). 

 Note that sodic amphibole occurs in crystal continuity with horn- 

 blende along path of albite veinlets. 



