56 



California Division of Mines and Geology 



Bull. i97 



Table 11. Chemical analyses of samples from the Bird Canyon Ledge (after Bowen and Gray, 1959, p. 32). 



ND = Not done. 



Samples G-21 through G-26 were analyzed by L. A Caetano, Ideal Cement Company. San Juan Bautista Samples G-42 through G-5S were analyzed by Abbot 

 A Hanks, Inc.. San Francisco. 1958. 



Most of the mass appears to be medium crystalline, 

 blue-gray limestone. Reserves probably aggregate 

 many millions of tons." Although the deposit is un- 

 developed, it appears worthy of some future sampling 

 and e.xploration in order to assess the potential re- 

 sources. It is accessible by dirt roads from the Salinas 

 \'alley. 



Bryan and Pearce-Twohy (Ideal Cement Com- 

 pany) deposit. Location: N'/; sec. 24 (proj.), 

 T. \i S., R. 4 E., M.D., 5 miles southeast of San Juan 

 Bautista; HoUister 1 S-minute quadrangle. Ownership: 

 Ideal Cement Company, 420 Ideal Cement Building, 

 Denver, Colorado 80202, holds mineral rights (1963). 



In 1927, Pacific Portland Cement Company (prede- 

 cessor to present owner) acquired the holdings of Old 

 Mission Portland Cement Company and intensively 

 prospected this deposit by core drilling and surface 

 sampling. Later (1941?), a quarry was developed to 

 provide limestone for renewed cement manufacture. 

 The plant was shut down between 1930 and 1941. The 

 plant closed again in 1943 but reopened in early 1947. 

 Shortly thereafter, the Bryan and Pearce- Iwohy 

 deposit became the principal source of limestone used 

 at the cement plant near San Juan Bautista.* 



The deposit consists of two adjacent masses of crys- 

 talline limestone associated with schist of the Sur Se- 

 ries and cut by intrusions of granitic rocks. An early 

 map of the deposit (Pacific Portland (>ement (Com- 

 pany, 1927, unpublished) shows the largest mass to be 

 northwest-trending and irregular in plan. It has a 



• Quarry operations ceased about 1973 (see Ideal Cement Company — San 

 Juan Bautista ftmtnote on p. 65). 



length of 2,300 feet and an average width of about 400 

 feet and is exposed through at least 550 feet of relief. 

 A smaller mass, roughly 600 by 500 feet in plan, is 

 situated immediately west of the main mass. Lime- 

 stone beds in the main mass generally dip 20° to 50° S, 

 being steeper to the south, and probably represent 200 

 to 300 feet of stratigraphic thickness. Apparently the 

 deposit is somewhat broken by faulting, as indicated 

 by the presence of breccia and fault gouge in the quar- 

 ries. The limestone is medium to coarsely crystalline, 

 white to blue gray, and uniform in appearance and 

 chemistry. The average analysis of 50 surface samples 

 and 500 feet of diamond drill cores, sampled and 

 analyzed by Ideal Cement Company, shows 51.1% 

 CaO, 1.4% MgO, 41.7% CO2, 4.1% SiOj, and 1.7% 

 FeaOa and AI2O3 (Bowen and Gray, 1959, p. 25). Based 

 on the dimensions of the limestone masses, limestone 

 reserves are estimated to be 8 to 9 million tons per 

 hundred feet of depth. Granitic intrusives and other 

 noncarbonate rock inclusions reduce reserve esti- 

 mates significantly, but to an unknown e.xtent. Bowen 

 and Gray (1959, p. 25) estimate total reserves to be 

 about 20 million tons; but, due to unfavorable strip- 

 ping ratios, part of the limestone is not recoverable by 

 surface methods. 



Development of the deposit is centered in two quar- 

 ry areas in the main limestone mass. According to 

 Oliver E. Bowen ( 1963, personal communication), the 

 largest quarry is situated along the crest of the main 

 east-trending ridge. In 1961, it was reported to be 

 about 1200 feet long by 200 to 300 feet wide. A smaller 

 quarry to the southeast covered an area roughly 800 

 feet long (east-west) by 100 to 200 feet wide. The 



