FRANCISCAN GROUP 



19 



FIGURE 8. Alta lying above intrusive serpentine contact on the 500 

 level in the Harry workings of the New Almaden mine. Serpentine 

 converted to silica-carbonate rock (sc) and cut by a few dolomite 

 veinlets. Alta contains larger fragments than in most places and 

 consequently shows sheared texture somewhat better. 



constituent. In most places the alta also contains 

 some of the other rocks of the Franciscan group, to- 

 gether with small pods and lenses of serpentine or 

 silica-carbonate rock. (See fig. 8.) 



Two processes seem to have operated together to 

 form the structures that are characteristic of the alta. 

 One is shearing, and as most of the serpentine bodies 

 in the mine area are sill-like the shears in the alta, 

 which are parallel to the intrusive contact, are also 

 closely parallel to the bedding. The other process is 

 compression perhaps due to the intrusion applied at 

 right angles to the bedding. These two processes op- 

 erating together have caused stretch ing of the indi- 

 vidual rock layers and a flowage of the shale. Where 

 thin layers of graywacke or tuff are interbedded in 

 the alta they commonly have been drawn out into iso- 

 lated lenticular pods forming a boudinage structure. 

 Where thicker beds of massive greenstone or gray- 

 wacke are included they, too, are broken and drawn 

 apart, but the larger disconnected pieces commonly 

 retain more angular shapes. As the alta grades from 

 highly sheared rock near the intrusive contact to the 

 less sheared normal Franciscan rocks, it is possible in 

 places to observe all transitions from alta that resem- 

 bles fault gouge to bedded rocks of the Franciscan 

 group. (See figs. 9, 10.) 



In mine workings the appearance of the alta is 

 striking because its texture is emphasized by the 

 varied colors of the rocks in it. The shale, which 

 predominates, is all jet black; pods of tuff are altered 

 to light cream-colored clays; and pods of serpentine 





FIGURE 9. Hand specimen of alta showing fragments of graywacke 

 and siltstone. This specimen is not typical of most alta ; it was 

 selected because the shearing has not obliterated all the original 

 bedding, as it has in more typical specimens. 



or silica-carbonate rock are generally green. The gray- 

 wacke, although not very light colored, is enough 

 lighter than the black shales to make a contrast. In 

 most surface exposures, however, the intense black 

 color of the shale has been lightened by weathering 

 to such a degree that the augenlike texture of the 

 alta is not conspicuous. 



< ' hcmlcul features 



Largely because of the prevalent jetblack color of 

 the shale in the alta, the writers suspected that it 

 might differ from the normal shale of the Franciscan 



FIGURE 10. Photomicrograph of alta. The coarser grained layers are 

 indurated and have yielded by breaking, whereas the finer material 

 has flowed. The same structural relations are commonly seen in 

 exposures, where dimensions are measurable in feet rather than in 

 fractions of a millimeter as in this thin section. 



