FRANCISCAN GROUP 



21 



conglomerates of the Franciscan group, were sought 

 but not found; serpentine pebbles also are lacking. 



Another variety of conglomerate was found as 

 float in several places in the district. It consists of 

 smooth round pebbles of black chert, !/fe to 1/2 inch in 

 diameter, closely packed in a siliceous matrix. The 

 rock as a whole is gray, very hard, and resistant, and 

 fragments of it are found in stream canyons far below 

 its outcrops. The rock is unusual in being largely 

 composed of a single variety of rock pebbles, which 

 apparently were derived from a distant land mass. 



ORGANIC AND CHEMICAL SEDIMENTARY ROCKS 



LIMESTONE 



Limestone constitutes only about 0.1 percent of the 

 Franciscan group in the New Almaden district, but 

 it is nevertheless important for several reasons. To 

 geologists the limestone is helpful because it forms 

 a discontinuous key-horizon that aids in deciphering 

 the structure of the area, and because it has yielded 

 the few fossils that give some local evidence regard- 

 ing the age of the Franciscan group. The limestone 

 also possesses economic interest because it has sup- 

 plied several limekilns in the district, and in the future 

 it may be used in making cement, for a similar lime- 

 stone is now used at the Permanente Cement Co. 

 plant a few miles to the northwest of the Almaden 

 area. 



The limestone exposed in the New Almaden district 

 is doubtless equivalent in part to the Calera lime- 

 stone of the Franciscan group (Lawson, 1914, p. 5, 

 22), which was observed within a few miles of the 

 northwest corner of the district in mapping the Santa 

 Cruz quadrangle (Branner and others, 1909). A 

 similar limestone is prominent also southeast of the 

 district, in the adjoining San Juan Bautista quadrangle 

 (Allen, 1946, p. 25). 



About 100 separate bodies of limestone are shown 

 on the geologic map of the district (pi. 1). They 

 occupy a belt that extends southeastward from the 

 hills south of Los Gatos to Longwall Canyon, beyond 

 which it swings northward and northwestward to 

 reach the vicinity of the Calero Reservoir, where the 

 limestone occurs in scattered blocks. (See fig. 56.) 

 Farther east, in the adjacent Morgan Hill quadrangle, 

 the same limestone crops out in more continuous ex- 

 posures 2 miles south of the mouth of San Bruno 

 Canyon. 



Nearly all the limestone bodies are comparatively 

 small, and the size of many of the smaller mapped 

 outcrops had to be exaggerated on the map to make 

 their position apparent. The most extensive out- 

 crops, none of which are quite 2,000 feet long, occur 



in two general areas one close to the southeast corner 

 of the district and the other near its western edge 

 and south of Los Gatos. The thickest body, which 

 crops out on Mine Hill about 800 feet southwest of the 

 modern furnace, appears to be about 10 feet thick, 

 but it may be isoclinally folded. Most of the lime- 

 stone masses that have been quarried are only about 

 50 feet thick, although in some of the quarries they 

 appear thicker because of repetition by faulting. 

 Many of the outcrops, however, appear to represent 

 beds less than 15 feet thick, and in a good many the 

 limestone forms isolated, roughly equidimensional 

 blocks less than 8 feet in diameter. Because of the 

 variation in thickness of the limestone from place to 

 place, it is believed to have been deposited as lenses, 

 of which only a few were as much as 50 feet thick; 

 these lenses, in turn, may subsequently have been 

 broken and pulled apart by erogenic movements to 

 form the smaller blocks. 



Megascopic features 



The limestone is one of the most easily recognized 

 rocks in the area. It forms some characteristic bold 

 outcrops that, because of their striking white color, 

 can scarcely be overlooked, and it gives rise to boul- 

 dery float that serves to indicate its presence on grassy 

 or wooded slopes. Probably, therefore, very few 

 outcrops of limestone are omitted from the geologic 

 map (pi. 1). The limestone shows some variation 

 from place to place, and it includes two principal 

 varieties, which seem to have been deposited at 

 slightly different times. 



The more widespread of the two varieties, which 

 is the older, corresponds to the typical Calera lime- 

 stone; it generally shows fairly well-developed bed- 

 ding, and in most exposures it contains gray or white 

 chert in elongate discontinuous thin lenses flattened 

 parallel to the bedding (see fig. 11). The poorly de- 

 veloped parting layers, which are from a few inches 

 to several feet apart, commonly contain a thin film 

 of shale and may be either fairly regular or stylolitic 

 on a small scale. In most outcrops these thin layers 

 provide the only record of any interruption in the 

 deposition of the limestone, but locally the limestone 

 occurs as thin lenses intercalated with tuffaceous and 

 calcareous shales. (See figs. 12, 13.) The color of the 

 freshly broken surfaces is generally black or dark 

 gray, but in some varieties is white or pink. The 

 darker varieties have a strong fetid odor, due to 

 petroliferous material and hydrogen sulfide. Most 

 of the lighter colored varieties contain minute Forami- 

 nifera, which appear to the unaided eye as small 

 transparent dark specks but can be seen under a hand 



