FRANCISCAN GROUP 



35 



FIGURE 25. Photomicrograph of calcareous tuff from the Franciscan 

 group. The fragments are various mafic rocks, most of which con- 

 tain some altered tachylite ; the matrix is ealeite (C). 



Megascopic features 



The appearance of outcrops of the tachylitic rocks 

 depend upon the grain size of the rock and the degree 

 of weathering. The finer grained rocks are generally 

 little weathered, moderately well exposed, massive, 

 and dark green; the more fissile varieties tend to part 

 much like a schist, giving rise to indistinctly bedded 

 outcrops. In hand specimen these tuffaceous rocks 

 appear massive, breaking with a hackly fracture and 

 possessing a distinctive but fine-grained texture that 

 is easily recognized though hard to describe. This 

 texture is characterized by irregular roughly elliptical 

 masses of greenish altered glass so closely packed in 

 a matrix of chlorite that they appear to have deformed 

 one another. Many of the elliptical masses contain 

 smaller flattened black or deep-green amygdules, and 

 almost all show concentric bands of various shades of 

 green. In the typical tuffs the fragments are about 

 1 mm in diameter, but these rocks grade to breccias 

 having fragments, dominantly of vesicular basalt, as 

 much as 6 inches in diameter. (See fig. 26.) In out- 

 crop the fragments of vesicular lavas are generally 

 weathered reddish brown, although where fresh they 

 are green to black. The breccias show only crude bed- 

 ding and sorting, and their having contained a glassy 

 matrix could hardly be recognized except in the fresh- 

 est specimens. 



Microscopic features 



Thin sections of the tachylitic tuffaceous rocks are 

 striking in appearance because of the relict textures 



imparted by the original glass, as is shown in figure 27. 

 The tachylitic glass has all been replaced by micro- 

 crystalline aggregates of doubly refracting minerals, 

 some of which are extremely fine grained and have not 

 been identified with certainty. Some of these altera- 

 tion products are chlorite, and some appear to be mi- 

 crocrystalline feldspar; none appears to be palagonite. 

 The few mineral grains found in the tuffs and appar- 

 ently of primary origin are crystals of augite and 

 very sodic albite. The deep-colored filling of the small 

 vesicles that can be seen with a hand lens is probably 

 nontronite, and other smaller vesicles are filled with 

 albite (An 2 ). A matrix of very fine grained, nearly 

 isotropic chlorite is generally present. The coarser 

 breccias contain, in addition to shards, tachylitic la- 

 pilli and glassy and diabasic volcanic fragments that 

 are similar in composition and microscopic texture to 

 other greenstones of the Franciscan group. Some of 

 the tachylitic rocks are extensively replaced by calcite, 

 and others contain veins and replacement masses of 

 albite associated with a little quartz. 



CHEMICAL COMPOSITION OF THE GREENSTONE 



Two new chemical analyses of greenstone of the 

 Franciscan group, together with three older ones, are 

 presented in table 8, which also includes some average 

 analyses of diabase and spilite for comparison. With 

 so few available analyses of these rocks, which are 

 altered in such various ways, it is unsafe to draw any 

 general conclusions regarding their composition; it 

 does seem advisable, however, to point out a few of 

 the problems awaiting further study. 



Mafic rocks associated with the eugeosynclinal ac- 

 cumulations of graywacke, black siltstone, and chert, 

 like the sedimentary assemblage of the Franciscan 

 group, in other parts of the world are commonly rich 

 in sodium, contain abundant albite, and are classed 

 as spilites. Pillow lavas, such as are common in the 

 Franciscan group, likewise, are in many places spilitic. 

 Largely because of these facts the greenstones of the 

 Franciscan group are regarded by many as spilites 

 (Reed, 1933, p. 83). In the New Almaden district 

 much of the greenstone contains albite, and the most 

 calcic plagioclase identified in the greenstones was 

 andesine. In other areas, however, the greenstones 

 are reported to contain only labradorite (Weaver, 

 1949, p. 113-114), or original andesine-bytownite lo- 

 cally altered to albite-oligoclase (Taliaferro, 1943b, 

 p. 145). Thus, from a consideration of the feldspars 

 alone, one might conclude that the greenstones of the 

 Franciscan group were normal diabases locally en- 



