FRANCISCAN GROUP 



27 



forming openings, which later were largely filled 

 with white quartz; small quartz crystals line cavities 

 that were not completely filled. In some specimens 

 the botryoidal surfaces are visible within the mass, 

 whereas in others, which have an equally well-devel- 

 oped botryoidal shell, the internal structures do not 

 indicate successive botryoidal layers. Conversely, 

 some chert breccias not exhibiting the surface struc- 

 tures show internal structures that appear to have 

 resulted from dehydration shrinking (fig. 18). These 

 evidences of dehydration, however, are found only in 

 small, generally isolated, masses of chert, and they 

 are not typical of either the rhythmically bedded or 

 massive chert lenses that account for 99 percent of the 

 chert in the Franciscan group. 



The light-colored chert that occurs mainly in the 

 lower part of the Franciscan group is exposed in small 

 outcrops generally less than 30 feet long and only a 

 few feet wide. These outcrops are too small and too 

 erratic to be shown on the map of the district, but 

 chert, interbedded with arkose, was observed in several 

 places along the ridge extending northwestward from 

 El Sombroso. This chert is massive, without shale 

 partings, and only in a few places does it contain 

 separation planes that indicate its bedding. Light- 

 colored quartz is the dominant mineral, but some 

 lenses contain a little oxidized pyrite, which has mis- 



led prospectors into believing that the siliceous lenses 

 were gold-quartz veins. No Radiolaria were noted 

 in any of these light-colored older cherts. 



Microscopic features 



In thin section the bedded chert is seen to be 

 largely a mixture of quartz and chalcedony, which 

 generally is clouded with red iron oxide dust. Some 

 varieties are composed of a very fine grained aggre- 

 gate of quartz, others contain chalcedony, and still 

 others have a matrix of silica that appears almost 

 isotropic but has an index of refraction slightly 

 greater than that of balsam. This material, which 

 apparently has puzzled others (Lawson, 1895b, p. 

 423) and deserves further study, is probably a crypto- 

 crystalline aggregate of chalcedony and quartz, with 

 overlapping crystals which tend to compensate each 

 other between crossed nicols. In thin section, clear 

 areas having the outlines of Radiolaria stand out in 

 contrast to the clouded matrix, and where the Radio- 

 laria are unusually well preserved, the spines and 

 mesh structures, as well as the general outlines of the 

 microfossils, can be distinguished (fig. 19). Most of 

 these clear areas consist of quartz a little coarser 

 grained than that composing the matrix, but some 

 consist of chalcedony, whose fibers may radiate from 

 one or more centers of growth. In a few thin sec- 



FIGURE 18. Vertical cut through part of a "cauliflower" of botryoidal chert showing internal structure. The 

 specimen is oriented as it occurs in nature. Dark is red-brown chert ; light is milky quartz. 



