FRANCISCAN GROUP 



15 



Because of the scarcity of exposures, the geologist 

 mapping in the Coast Ranges must often rely on the 

 character of the residual soil, and therefore a few 

 comments on the soils developed on graywacke may 

 be of value. Where the graywacke is highly feld- 

 spathic, the soils are light buff or tan and differ in 

 color from the reddish soils yielded by most green- 

 stones. Although we found a few areas of altered 

 tuffaceous greenstones that also yielded light-colored 

 soils, these were so small that we believe mapping 

 areas mantled with light-colored soils as graywacke 

 will cause only a very occasional error. Where the 

 graywacke contains abundant fragments of green- 

 stone, it gives rise to reddish soils identical in color to 

 some soils derived from tuffaceous or massive green- 

 stone. In areas so mantled the geologist should rely 

 on the small rock fragments that can commonly be 

 found in the soil ; in their absence no criteria is really 

 reliable, but we found that soils developed on gray- 

 wacke were generally more gritty than those on green- 

 stone, probably owing to the persistence of quartz. 



Megascopic features 



The most consistent features of the graywacke are 

 its dirty appearance and its high content of feldspar, 

 which normally slightly exceeds quartz in amount. It 

 contains highly angular grains, which are generally 

 monomineralic, and somewhat more rounded rock 

 fragments. The monomineralic grains are mostly of 

 feldspar and quartz and between 0.25 and 1.0 mm in 

 diameter, but a few detrital grains of minerals of the 

 epidote group and zircon are commonly present. Al- 

 though all the graywackes contain rock fragments, 

 the proportion is quite variable, ranging from a few 

 percent to three-fourths of the rock. Most of the 

 fragments are of basic lavas, but fragments of shale 

 are generally present and locally abundant. Carbon- 

 ized wood fragments, such as are abundant elsewhere 

 in the Franciscan group, are uncommon in the New 

 Almaden district. The indeterminate matrix is dark 

 colored, and ranges in amount from thin films be- 

 tween closely packed grains to perhaps as much as 

 20 percent. Where the matrix is highly siliceous, as 

 in much of the more feldspathic graywacke, the frac- 

 tures tend to go through the grains; where it is more 

 chloritic or clayey, however, the breaks go around the 

 grains. Xone of the rocks, however, contain more 

 than a little clay, and none contain an appreciable 

 quantity of red ferruginous oxides. The color gen- 

 erally is some shade of gray or light green, but where 

 the graywacke is weathered, as in most outcrops, it 

 ranges in color from dark gray to buff or light tan 

 and locally is reddish. 



FIGURE 5. Photomicrograph of feldspathic graywacke of the Francis- 

 can group. Crossed nicols. Angularity of grains and lack of sort- 

 Ing are typical. Grains are chiefly plagioclase (pi) and quartz (Q). 



Microscopic features 



Enough thin sections were studied to indicate that 

 the graywacke varies widely in relative proportions 

 of component minerals and in amount of matrix, but 

 many more sections would have to be studied before 

 definite limits could be assigned to the variations. 

 (See figs. 5, 6.) All the sections contain angular 

 grains of sodic plagioclase, quartz, and rock frag- 

 ments, separated by a fine-grained matrix composed 

 of the same materials and recrystallized fine-grained 



FIGURE 6. Photomicrograph of lithic graywacke of the Franciscan 

 group. Crossed nicols. Contains monomineralic grains of plagio- 

 clase (pi) and quartz (Q), and grains of greenstone (G), altered 

 tachylite (T), argillite (A), and chert (C). 



