78 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



this time on is well known. Petroleum geologists, 

 however, have a habit of classing the older rocks as 

 granitic and metamorphic basement or Franciscan com- 

 plex, and consider them only as much as seems re- 

 quired to determine possible oil-bearing areas and 

 structures. What detailed knowledge there is of the 

 older rocks of the Coast Ranges is largely a result of 

 work by geologists connected with various universities 

 or with State or Federal surveys engaged in the 

 search for economic deposits commonly associated 

 with the older rocks, notably the ores of quicksilver, 

 chromium, and manganese. 



In the New Almaden area the post-Franciscan rocks 

 do not cover large areas, and important structures 

 were developed before any younger rocks were de- 

 posited; many of the structural features, therefore, 

 must be deduced from the attitudes and distribution 

 of the rocks of the heterogeneous Franciscan group. 

 When this has been done, it becomes clear that these 

 older structures have affected profoundly the subse- 

 quent deformational pattern of the younger rocks in 

 the district, and this also seems to be true in other 

 parts of the Coast Ranges (Clark, B. L., 1932, p. 385- 

 401). Consequently, the study of these older struc- 

 tures is of vital importance in fully understanding 

 even the later structure and geologic history of the 

 Coast Ranges. 



A brief summary of the previous geologic studies 

 of terrains underlain by rocks of the Franciscan group 

 is desirable to orient the reader as to the present 

 knowledge of these older structures. Such areas in 

 the central Coast Ranges were studied largely from 

 a lithologic standpoint near the turn of the century, 

 and in the following decade the major geologic struc- 

 tures of limited areas were briefly described in three 

 geologic folios of the U.S. Geological Survey (Fair- 

 banks, 1904; Branner and others, 1909; Lawson, 1914). 

 During the last 20 years much larger parts of the 

 Coast Ranges have been ably studied by Taliaferro 

 (1943a, b), of the University of California, and many 

 students working under him, but although several ex- 

 cellent summary reports on this work have been pub- 

 lished, much of the more detailed structure remains 

 undescribed. A different approach to the structure of 

 the Coast Ranges has been a synthesis of the available 

 published data by such eminent students of the geology 

 of the Coast Ranges as Willis (1927, p. 34-37; 1946, 

 p. 1885-1886) and Reed (1933, p. 11-14, 27-59, 86-88). 

 Some European geologists (Kober, 1925, p. 139-144) 

 have compared the Franciscan sedimentation and de- 

 formation to that of major synclinal areas elsewhere 

 in the world. As a result of all these varied ap- 



proaches, as Taliaferro (l43b, p. 151) aptly points 

 out, "Almost every assert ion regarding the fundamen- 

 tal control of Coast Range structure has been met with 

 a contradiction." Some geologists have emphasized 

 the dominance of folding over faulting whereas others 

 have believed that the Coast Ranges were comparable 

 to the Alps in containing major overt hrusts and "mo- 

 bile belts". Some (Clark, B. L., 1!>:5<>. p. 747-M'v 

 1935, p. 1026-1034) have considered the Coast Ra> 

 to be fault blocks which periodically move up or down, 

 whereas others, though also believing them to be broken 

 into blocks, considered the dominant movement along 

 the bordering faults to be relative northwestward 

 shifts of the blocks on the southwest side of each fault. 

 Obviously, then, much remains to be done before the 

 older structures of large parts of the California Coast 

 Ranges are understood. The detailed mapping of the 

 New Almaden district, according to the methods de- 

 scribed, (p. 8-9) reveals structures that lead to a rea- 

 sonable analysis for this limited area; but whether 

 they are representative of typical structures of the 

 Coast Ranges will not be known until the results of 

 geologic study of areas of Franciscan rocks in a far 

 larger part of the Coast Ranges have been published. 



METHODS USED IN DETERMINING THE STRUCTURES 



The difficulties encountered in finding, mapping, and 

 explaining structures of the rocks of the Franciscan 

 group are due partly to the generally poor exposure* 

 and partly to lithology and structure. Lithologically. 

 the sequence of interbedded greenstones and feld- 

 spathic sedimentary rocks varies greatly from place 

 to place, yet few individual beds or groups of beds 

 possess distinctive features by which they can be cor- 

 related across even short intervals. Coupled with this 

 is the general lack of fossils in the rocks. As a result, 

 it is not generally possible to set up a "standard sec- 

 tion" before completion of the a real mapping, nor can 

 one rely on fossils for aid in correlation. These diffi- 

 culties due to lithology and lack of fossils would not 

 be so insurmountable were it not for the complications 

 added by the way the Franciscan group has responded 

 to deformation. The rocks have yielded by shattering, 

 or by crumpling accompanied by extensive rock flow- 

 age, to such a degree that nearly every part of the 

 group is broken and deformed. Consequently, an iso- 

 lated exposure may show an attitude that departs 

 widely from the attitude prevalent in the surrounding 

 area, and one cannot rely on the significance of any 

 local attitude observed in the field. Furthermore, the 

 rocks have not developed cleavage or systematic linear 

 elements by which one may determine the direction of 



