1978 



Limestone in the Coast Ranges 



59 



feet of relief. Medium- crystalline, blue-gray, some- 

 times banded or variegated limestone is the chief rock 

 tvpe. White medium-crystalline dolomite exists in 

 thin bands near the north and south margins of the 

 mass. The limestone is farther from the San Andreas 

 fault zone and is generally less brecciated than other 

 deposits in the Garner-Harris group. Development is 

 limited to several prospect cuts from which the owner 

 has collected 20 samples for analysis (see DT and D T 

 2 series in table 1.^). With the exception of one dolo- 

 mite sample ( 012 1 ) , the analyses show an average of 

 about 51% CaO and 4% SiO,. Total carbonate rock 

 reserves are estimated to be no more than 1.5,000 tons 

 per foot of depth. Selective mining would be necessary 

 to develop the deposit, and recoverable reserves may 

 be less than indicated. 



West Boundary deposit. A poorly exposed mass 

 of limestone trending N 70° W is situated a few hun- 

 dred feet northeast of the Dry Trough deposit. It ex- 

 tends over an area 1,700 feet long by a maximum of 250 

 feet wide. Smaller bodies of carbonate rock lie close by 

 to the southwest and east. The limestone is mainly off 

 white, medium to coarse crystalline, and partly brec- 

 ciated. Although outcrops are scattered, prospect cuts 

 and stripping partly define the mass. In recent years, 

 Howard Harris has collected and analyzed 36 samples 

 from this deposit (see samples of WB and G series, 

 table 13). Chemical analyses indicate that the lime- 

 stone contains variable amounts of silica and little 

 magnesia (two exceptions). Total reserves appear to 

 run about 12,500 tons or less per foot of depth. 



Blue deposit. This small lenticular mass trends N 

 70° \\ and lies just east of the Dry Trough deposit. 

 Maximum dimensions are about 700 feet by 125 feet. 

 It consists of light-gray to blue-gray, medium-crystal- 

 line limestone containing some thin bands of white 

 dolomite. Development is limited to several shallow 

 prospect cuts from which 15 samples were obtained 

 for analysis (B and B.A series, table 13). Estimated 

 reserves are less than 5,000 tons per foot of depth. 

 Considering the variable chemical composition of the 

 samples and apparent small reserves of limestone, the 

 deposit would not appear to be of significant commer- 

 cial value. 



Reference: Bowen ond Gray, 1959, p. 33. 



Hamilton deposit. Location: N'/; sec. 23, T. 14 S., 

 R. 5 E., M.D., 10 miles south of Hollister; Gonzales 

 15-minute quadrangle. Ownership: Mrs. A. E. Hamil- 

 ton, Cienega Road, Hollister (1962). 



This deposit includes three closely grouped lenses 

 of limestone, each of which has been developed to 

 some extent. There is .some confusion in the literature 

 regarding ownership, operations, and locations. Lime- 

 stone property in sec. 23 has been owned at various 

 times by U. G. Harlan; San Benito Lime Company 

 (Connelly and Kruse); Archer Lime Company (D. 

 McPhail); Hamilton, Fontaine, and Temple; U'. R. 



Fontaine; and Marie Mayries. From 1890 to about 

 1907, limestone was quarried and burned locally by U. 

 G. Harlan and probably even earlier by San Benito 

 Lime Company (Bradley and Logan, 1919, p. 340, 342; 

 Averill, 1947, p. 51-52). The San Benito Lime Com- 

 pany deposit apparently was acquired about 1925 by 

 Hamilton and associates and worked as a source of 

 lime rock and agricultural limestone from 1930 to 

 1932. During the 1950s and early 1960s, A. E. Hamil- 

 ton did considerable development work but was una- 

 ble to realize commercial production in recent years. 

 At the time of his death in 1962, a crushing-screening 

 plant, located just east of the main lens, was well to- 

 ward completion. A jaw crusher at the plant has a 

 capacity of 200 tons per hour. 



The Hamilton deposit is situated on a 2,83 1-foot hill 

 just north of Harlan Creek. It consists of three elon- 

 gate lenses extending half a mile north and slightly 

 east of the center of sec. 13. Perhaps the largest lens 

 occupies the north half of the deposit and is about 

 1,300 feet long by a maximum of 275 feet wide. A few 

 hundred feet to the south are two adjacent, parallel 

 lenses, each about 1,000 feet long by 100 to 150 feet 

 wide. Smaller masses of carbonate rock exist to the 

 east and west. The various lenses consist mainly of 

 white, extremely coarse-crystalline limestone associat- 

 ed with white to cream, medium-crystalline dolomite, 

 particularly near the margins. The country rock and 

 the numerous dikes that cut the lenses are granodior- 

 ite, now decomposed. 



The north lens trends north-northeast, is slightly 

 arcuate in plan, and appears to dip steeply to the east. 

 It is developed at its south end by a small quarry, a 

 275-foot tunnel, and prospect cuts near the summit of 

 the hill. The limestone contains scattered graphite 

 crystals and small amounts of white, finely fibrous 

 tremolite asbestos distributed along shear planes. A 

 typical sample of the coarse-crystalline limestone, con- 

 taining minor graphite and tremolite, was analyzed by 

 Lvdia Lofgren in 1963. It contained 53.6% CaO, 1.53% 

 MgO, 0.56% SiO,, 0.06% A1;0„ 0.05% Fe,0„ 0.01% 

 P,0,, and 43.62% ignition loss. The lenticular body is 

 locally dolomitic at its northeast end and west mar- 

 gins. Estimated reserves of limestone are on the order 

 of a million tons. 



Two parallel lenses constitute the south half of the 

 deposit. The east lens was developed by a small quarry 

 near its north end. Here the limestone is white but 

 locally discolored, and some dolomite is present along 

 the west margin. The lens strikes X 10° E and dips 

 about 60° E. In addition to the quarry, the body is 

 developed by several prospect cuts. 



Immediately to the west is a parallel, somewhat 

 thinner lens that appears to have an average width of 

 less than 100 feet. This body may be a detached exten- 

 sion of the main mass a few hundred feet to the north. 

 In the quarry exposures at the south end of the lens, 

 the limestone appears to dip steeply east and is trun- 

 cated on the down-dip side by granitic rocks. The 



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