FRANCISCAN GROUP 



33 



contains unreplaced feldspar that gives sharp extinc- 

 tions so that its composition can be determined. Albite 

 twinning is prevalent ; zoning was observed in some 

 sections but is not common. Although the feldspars 

 of these rocks are invariably too soda rich to be nor- 

 mal for such mafic rocks, the individual crystals in 

 most specimens show no ragged borders, irregular 

 patches, or other features suggesting that the sodic 

 plagioclase has replaced a more calcic feldspar. In 

 the coarser grained lavas and phenocrysts the plagio- 

 clase is invariably albite (An 2 . 7 ). In a little of the 

 aphanitic lava plagioclase as calcic as An 40 was noted, 

 but in most the plagioclase is near albite. 



Pyroxenes are important constituents of the green- 

 stones of the Franciscan group, making up from 20 

 to 35 percent of their volume. They deserve more 

 thorough study than has been made for this report, 

 but enough work has been done to indicate that they 

 belong to several varieties. The optic-axial angle 

 (2F) of a few crystals of augite was determined by 

 Dr. C. M. Swinney, of Stanford University, by means 

 of universal stage. The angles noted in 2 sections 

 ranged from 36 to 43, indicating subcalcic augite 

 (Benson. 1944, p. 111-118), and estimates of the optic- 

 axial angle of other pyroxenes in' thin sections we 

 examined indicate that this is the most common va- 

 riety. Another variety of pyroxene, found only as 

 stubby euhedral crystals in the finer grained lavas, 

 shows the purplish tints of titan-augite. Still a third 

 variety having a plumose or radial habit did not yield 

 sufficiently good interference figures to permit a reli- 

 able estimate of the optic-axial angle, which appears 

 to be large, and this pyroxene remains undetermined. 



Basaltic glass is believed to have been abundant 

 originally in much of the lava, although it now has 

 everywhere been completely converted to a mixture 

 that includes chlorite, probably nontronite, and other 

 fine-grained minerals not determined. The material 

 believed to have once been glass has a mottled appear- 

 ance, and encloses a myriad of skeleton crystals and 

 microlites of magnetite and plagioclase. 



The most abundant secondary minerals are chlorites, 

 which commonly make up from 15 to 25 percent of 

 the altered lavas. They replace the original basaltic 

 glass and augite and form a minor part of the saus- 

 suritic aggregates replacing plagioclase. Clinochlore 

 is the predominating variety, and although several 

 other varieties have been distinguished, they have not 

 been sufficiently studied to warrant assigning specific 

 names to them. Epidote and clinozoisite are wide- 

 spread in fine-grained aggregates replacing plagio- 

 clase, and in some rocks they occur as secondary eu- 

 hedral crystals large enough to be readily identified. 



Both quartz and calcite locally replace much of the 

 groundmass of the lavas, or fill vesicles in them, but 

 these minerals are more common in veinlets. Where 

 the age relation between the two minerals is clear, the 

 calcite is invariably the later. 



PYROCLASTIC ROCKS 



Pyroclastic rocks, including breccias, tuffs, and al- 

 tered tachylitic tuffs, make up at least a fifth of the 

 greenstone of the New Almaden district. They are 

 commonly much altered and are recognized chiefly by 

 their fragmental or bedded character coupled with 

 their mafic composition. They are exposed principally 

 in two bands. One band, which is narrow but per- 

 sistent, extends eastward from a point about 1 mile 

 north of the Calero Reservoir through the eastern part 

 of the Santa Teresa Hills, and continues beyond the 

 eastern border of the district. The other, which is 

 broader but less continuous, extends from a point on 

 the west edge of the district just south of Los Gatos 

 and crosses the central part, passing through the 

 Guadalupe and New Almaden mine properties, and 

 continuing nearly to the southeast corner. 



The pyroclastic rocks are divided into two groups 

 for description. In one group the main original con- 

 stituent was obviously tachylite, whereas in the other 

 group it was not. 



NONTACHYLITIC TUFFS AND BRECCIAS 



The nontachylitic tuffs and breccias make up most 

 of one thick body of greenstone that extends from the 

 vicinity of the Almaden Reservoir, in Almaden Can- 

 yon, northwestward through the central part of Mine 

 Hill and along the north slope of Los Capitancillos 

 Ridge to the vicinity of the Senator mine. As these 

 rocks are readily weathered, they are exposed on the 

 surface only in sharply incised canyons and roadcuts. 

 In good exposures, however, particularly those in the 

 workings of the New Almaden mine, the tuffs are seen 

 to be generally interbedded with black shale, but in 

 some places they are interbedded with graywacke, 

 chert, and lavas of the Franciscan group. In the New 

 Almaden mine a good exposure of alternating thin 

 beds of tuff and shale can be seen in the upper part 

 of the raise that extends from the 700 level to the Far 

 West stope, and a well-exposed thick section of tuff 

 containing only a little shale is cut by the Day tunnel 

 just south of the crosscut to the Santa Rita shaft 

 (pi. 4). 



Megascopic features 



The appearance of the nontachylite breccias and 

 tuffs is so varied as to defy simple description. They 

 exhibit a wide range of colors and textures, and they 

 have suffered varying degrees and kinds of alteration, 



