FRANCISCAN GROUP 



31 



nal glass is replaced by albite, indicating a type of 

 spilitic alteration. Very low grade regional metamor- 

 phism may be responsible for extensive chloritization, 

 and may have also caused the widespread irregular 

 veining of the rocks with quartz and calcite. The 

 greenstones of small areas have been still further meta- 

 morphosed to form amphibolites, which are discussed 

 with the other more highly metamorphosed rocks of 

 the Franciscan group on pages 39^40. 



In the descriptions given in the following para- 

 graphs the greenstones are grouped according to 

 whether they were originally lava flows or pyroclastic 

 rocks. 



IAVAS 



The greenstones derived from mafic lava flows are 

 more abundant and more widely distributed than those 

 derived from pyroclastic rocks. The lavas differ widely 

 in texture, and their groundmasses range from holo- 

 crystalline to glassy. The coarser varieties doubtless 

 grade into the aphanitic varieties in many places, but 

 owing to the poor exposures such transitions were ob- 

 served only in individual pillows, some of which have 

 diabasic centers and glassy rims. An attempt to sub- 

 divide the lavas into diabasic and aphanitic textural 

 types was made during the field mapping in hope of 

 establishing a stratigraphic sequence, but this attempt 

 failed because of the intermixing of the varieties and 

 the scarcity of exposures. It was evident, however, 

 that the aphanitic rocks are the most abundant, and 

 that the diabasic rocks occur most commonly within 

 the larger masses of greenstone. 



The character of the exposures of the massive green- 

 stone varies with differences in the original character 

 of the lava and in its mode of alteration. The best 

 exposures are those formed by lavas exhibiting pillow 

 structure, for these are among the most resistant rocks 

 in the district ; in several places they form rugged 

 cliffs. Bold outcrops are also found where the lavas 

 are silicified, but generally these are small and errati- 

 cally distributed. The more widespread diabasic and 

 amygdaloidal lavas are well exposed in some canyon 

 bottoms, but elsewhere they underlie large areas that 

 contain only widely spaced subdued outcrops, sur- 

 rounded by a characteristic red-brown soil containing 

 fragments of the altered greenstone. 



Megascopic features 



The unweathered greenstones are dark green to 

 black, but in outcrop they are generally weathered 

 and reddish brown. The coarser varieties are readily 

 recognized, for they have diabasic textures in which 

 the tabular crystals of feldspar, oriented at random, 

 are large enough to be distinguished with the naked 

 eye. The feldspar is usually the only mineral that can 



be identified megascopically, but because of its gener- 

 ally altered character, it is greenish white and opaque 

 rather than glassy; and although it is plagioclase, its 

 twinning is not always visible. In the finer grained 

 lavas the textures are not apparent and individual 

 minerals cannot be determined even with a hand lens. 

 These rocks, however, commonly show such features 

 as pillow structure, flow banding, and vesicles, which 

 aid in their recognition. Vesicles, where present, are 

 generally filled with either calcite or deep-green non- 

 tronite. Irregular veins of both quartz and calcite are 

 abundant in some of the greenstone. 



Microscopic features 



Although thin sections of the lavas reveal only minor 

 variation in kind or proportion of the primary miner- 

 als, they show wide differences in texture and grain 

 size and in the character of their secondary minerals. 

 (See figs. 20-23.) The principal primary minerals are 

 sodic plagioclase and either subcalcic augite or titan- 

 augite ; accessory minerals are magnetite, ilmenite, leu- 

 coxene, sphene, rutile, and apatite. Mafic glass was 

 originally present in most varieties, but has everywhere 

 been altered to chlorite. Olivine was once present in 

 a few of the greenstones, but it has been completely 

 replaced by either antigorite or iddingsite. Other 

 common secondary minerals identified are albite, chlo- 

 rites, epidote and clinozoisite, quartz, calcite, and non- 

 tronite. A more thorough study of some of the finer 

 grained aggregates of secondary minerals would no 

 doubt reveal other species. 



The textures of most of the greenstones as seen in 

 thin section are well preserved in spite of the advanced 

 stage of alteration. Most of the coarse-grained lavas 

 are holocrystalline ; the coarsest contain plagioclase tab- 

 lets 3 mm long and pyroxene prisms a little longer. 

 In the aphanitic rock, however, glass was an impor- 

 tant constituent, amounting in some to as much as 40 

 percent. Among the holocrystalline rocks an inter- 

 granular texture is more common than a truly diabasic 

 texture; in the aphanitic rocks the texture is widely 

 varied and no single kind of texture variety predomi- 

 nates. Porphyritic textures are uncommon, although 

 a little of the greenstone has an aphanitic groundmass 

 containing scattered phenocrysts of albite as much as 

 4 mm long. 



The plagioclase, which before secondary alteration 

 amounted to from 30 to 60 percent of the lavas, is 

 generally euhedral in form, although in those rocks 

 that have undergone severe deuteric alteration re- 

 newed growth of plagioclase has formed crystals that 

 are somewhat sutured and interlocked. Much of the 

 plagioclase is partly saussuritized or replaced by chlo- 

 rite, epidote, or calcite, but nearly all the greenstone 



