26 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



Megascopic features 



The cherts of the Franciscan group show considera- 

 ble diversity of character, probably because they do 

 not all have the same origin. Davis (1918, p. 235- 

 432) has well described the field occurrences and 

 lithology of the various kinds of chert in other parts 

 of the Coast Ranges, and because his descriptions 

 could apply equally well to the cherts of the New 

 Almaden district, only the more significant features 

 will be repeated in this report. The typical chert of 

 the Franciscan group is well bedded, generally 

 rhythmically bedded, and red or green. A more un- 

 usual kind, locally associated with the well-bedded 

 chert, exhibits botryoidal surfaces and distinctive 

 internal structures which are described below. Less 

 abundant but more typical of the older part of the 

 Franciscan group is a massive nearly white chert, 

 which also occurs sporadically in the younger part of 

 the sequence. Other light-gray to white chert occurs 

 as thin lenses or nodules in the limestone of the Fran- 

 ciscan group, and is not separated from the limestone 

 on the maps accompanying this report. Still other 

 chertlike rocks clearly formed by silicification of mafic 

 tuffs are not considered to be true cherts and are 

 included with the greenstone on the geologic maps. 



The most striking feature of the good outcrops of 

 bedded chert is their ribbed appearance, due to rhyth- 

 mic bedding of layers of chert and shale of the same 

 color, either red or green. The layers of chert are 

 generally between 1 and 2 inches in thickness, whereas 

 the shale partings are in most places not more than 

 a quarter of an inch thick and are commonly much 

 thinner. The individual chert layers are blunt-ended 

 lenticules that extend at most only a few tens of feet; 

 some of these layers vary in thickness, the bulges of 

 one layer commonly being compensated by thinning 

 in an adjacent layer. Sharp chevron folds a few feet 

 across and minor faults at low angles to the bedding 

 planes are common. At sharp bends in the folded 

 cherts as at the axes of chevron folds, the chert is 

 likely to l>e appreciably thicker but unfractured, sug- 

 gesting that the deformation may have taken place 

 while the chert was still plastic (see fig. 17). 



Fractures, not related to the folds and found in 

 almost all the \\ell U-dded chert, form two systems 

 of joints, both of which are nearly perpendicular to 

 the lidding and commonly intersect on the bedding 

 surfaces at angles of alx>ut 120. In many places 

 these fractures are small faults that make little steps 

 on the bedding surface; in other places no displace- 

 ment is noticeable. Some cracks are open near the 

 upper lidding surface like mud cracks; elsewhere they 



Floruit 17. Sharp folds in bedded chert of the Franciscan group. 

 The absence of fractures through the axial parts of the folds li 

 typical, and has been cited as evidence of folding during submarine 

 slumping before litbincation. 



are tight. Locally, the cracks are filled with silica, 

 which resists weathering and forms ribs; or they are 

 filled with calcite, which weathers away and forms 

 grooves. Surfaces of the chert broken across the bed- 

 ding are generally smooth and conchoidal; bedding 

 surfaces are slightly pitted or pimply. Kxamination 

 under the hand lens shows the minor surface irregu- 

 larities to be due, in part at least, to the unequal 

 weathering of small grains of clear silica that has 

 replaced, or filled in, the Radiolaria that abound in 

 some of the cherts. 



The chert exhibiting botryoidal surfaces is uncom- 

 mon in the district; it is exposed only in a group of 

 outcrops half a mile north of the summit of Fern 

 Peak and is found as float on a small knob about a 

 quarter of a mile south of Tulare Hill. Its color at 

 both places is brownish red with greenish patches. 

 The outcrops near Fern Peak are small, not exceed- 

 ing _'<> feet in diameter and generally much smaller, 

 and are roughly eqoidimensional. These are made up 

 of smaller bodies that are shaped somewhat like a 

 cauliflower. Kach cauliflower has a nearly flat lower 

 surface that shows some flattened l>otrvoidal bumps 

 and a roughly hemispherical upper surface on which 

 arc sii]>cr|>osed smaller hemispheres or more irregular 

 flattened bumps and ridges not more than a couple of 

 inches across. The bumps are separated by sharp 

 Km wide V-shaped groove-, and in most places they 

 appear to have IHHMI packed on top of each other so 

 that the surface closely resembles that of certain kinds 

 of -palter cones. Kqually distinctive are the internal 

 structures of the botryoidal chert (fig. IS). These 

 feathery arcuate structures appear to have resulted 

 from shrinkage of an originally very hydrous gel 



