Area: 1,328 square miles.
Population: 100,588 (1920 census).
Location: West-central portion of state.
"Santa Clara County reported a mineral output for 1919 of $1,048,571, as compared with the 1918 figures of $1,759,568, the decrease being due to potash, manganese and quicksilver.
"This county, lying largely in the Coast Range Mountains, contains a wide variety of mineral substances, including brick, chromite, clay, limestone, magnesite, manganese, mineral water, petroleum, quicksilver, soapstone, and miscellaneous stone. It stood second in quicksilver yield for the year.
"In twenty-second place, commercial production for 1919 was as follows:
(Headings for the information below are: Substance, Amount, and Value.)
Brick, 7,250 M., $65,000
Clay, 2,532 tons, $2,232
Magnesite, 10,912 tons, $128,924
Manganese, 102 tons, $3,321
Petroleum, 16,724 bbls., $26,695
Quicksilver, 3,012 flasks, $271,762
Stone, miscellaneous, ---, $73,237
Other minerals,* ---, $477,400
(Total value) $1,048,571
(* Includes limestone, mineral water, and potash.)
Abstract (excerpts from)
Santa Clara County Mineral Production
"Santa Clara, one of the central Coast Ranges counties of California, embraces 1,305 square miles at the southern end of San Francisco Bay. The Santa Clara Valley, trending northwestward through the center of the county, is flanked by the Santa Cruz Mountains on the west and by the Diablo Range on the east.
The oldest rocks found within the limits of the county are included in the Franciscan-Knoxville group of Upper Jurassic (?) age. These rocks form the largest mappable (sic) unit in the area and are exposed in the mountains on both sides of the range and are found also in the Santa Cruz Mountains. Miocene sediments border on the northern Diablo Range. Marine and continental Pliocene sediments crop out in patches along both margins of the Santa Clara valley. The valley floor is composed of a vast accumulation of sand and gravel of Quaternary age. Tertiary volcanic rocks occur locally on both sides of the valley'."
"The Permanent Cement Company constructed their plant on Permanent Creek in 1939 as a two-kiln unit. Expansion of the plant continued during the following World War II until the annual output reached 7,000,000 barrels in 1952. Five kilns are now in operation (circa 1954). Clay for cement is obtained from a bed of altered andesite which crops out on a mountain spur between the limestone quarry and the cement plant. This material is supplemented by lateritic clay and pyrite sinter obtained outside the county. Limestone is obtained from the highly fractured and contorted Calera member of the Franciscan formation (Jurassic ?)'."
"Occurrences in limestone have been noted in formations ranging from Jurassic to Recent in age. The only commercial production, however, has been from the Calera limestone member of the Upper Jurassic (?) Franciscan formation; discontinuous outcrops of limestone are found along the east slope of the Santa Cruz Mountains. The Franciscan limestone is interbedded with chert and the principal production from this formation has been made at Black Mountain for the Permanent cement plant. Limestone has been calcined to lime at both Guadalupe and Los Gatos Creeks, although no such activity has been reported since 1913. Shells from southern San Francisco Bay, processed at Alviso, have been a source of lime in recent years'."
"Serpentine has been quarried and fused with phosphate rock. The fusion-product was sold as a fertilizer.
"Dimension sandstone was quarried on the south flank of the Santa Teresa Hills from 1900-05. This stone was used in construction of campus buildings at Stanford University, and for many other buildings in California'."
Limestone (in Santa Clara County)
"Limestone is composed chiefly of the mineral calcite (calcium carbonate, CaCO3). Most limestones are impure; the impurities include magnesium carbonate (dolomite, MgCO3); silica, SiO2 (as quartz or chert); iron oxide, Fe2O3; clay (aluminum silicate); or organic matter. Travertine is a variety of limestone deposited from solution by groundwater or springs.
"Limestone is extensively used in chemical and manufacturing industries, in cement, in metallurgy, in agriculture, in the building industry, and in road construction. Many of these uses have been described in publications of this Division (Ballou, 1951; Bowles and Jensen, 1949; Logan, 1947, pp. 180-200).
"Occurrences of limestone have been noted from Santa Clara County in rocks ranging from Jurassic to Recent in age (Logan, 1947, pp. 310-317). The only production of commercial limestone, however, has been made from a member of the Jurassic (?) Franciscan formation. These limestone beds were named Calera limestone by Lawson (1914) who originally studied them at Calera Hill, near Rockaway Beach, San Mateo County. Discontinuous outcrops of limestone similar to the Calera in the Franciscan formation can be traced southeastward into Santa Clara County along the west side of the Santa Clara Valley as far south as Gilroy. These outcrops follow the trend of the San Andreas and associated faults through the area. Some geologists have considered the Calera limestone to be discontinuous faulted segments (Walker, 1950). Quarries have been opened and production has been made in Santa Clara County at the following localities: Permanente Creek near Black Mountain, southeast of Los Gatos, near the Guadalupe mine, in the Santa Teresa Hills, and in the hills southwest of Gilroy. The limestone is a fine-grained, high-lime rock, much of which is interbedded with, or contains lenses of, chert. The presence and distribution of the chert usually governs the method of quarrying, and hence determines the commercial value of the particular deposit.
"The occurrence of small limestone lenses in sedimentary rocks of Tertiary age have been noted by Crittenden (1951) and Gilbert (1943). These lenses are found on the east side of the Santa Clara Valley. No commercial production has been reported.
"A deposit of travertine of Recent age in the southwestern part of the county has been described by Allen (1946, pp. 47, 72) who stated:
"'A tributary to Hatfield Canyon has built up its bed for over a half a mile with travertine terraces. The lime-rich waters are derived from a series of small springs which are aligned in a northwesterly direction in the headwaters of the tributary. The terraces consist of cemented boulders and blocks of sandstone at at (sic) the base, overlain by alternating pools and falls whose lips are made up of buff to gray travertine.'
"Allen also mentions the production of a small tonnage of Franciscan limestone from the area north of La Brea Creek."
Lime (in Santa Clara County)
"Lime, calcium oxide (CaO), is formed as a result of heating limestone (CaCO3), to a temperature at which the carbon dioxide (Co2) is removed as a gas. Lime manufacturing is an ancient industry and lime is widely used in the building trades, in agriculture, and in numerous chemical processes (Bowles and others, 1941).
"Although the Spanish padres may have produced lime during the Mission period, the first commercial lime on record in Santa Clara County was produced in a kiln near the Guadalupe mine about 1864 (Irelan, 1888, pp. 543-546). The Guadalupe limestone quarries and kiln were active for about 30 years.
"Limestone was quarried southeast of Los Gatos as early as 1886 and was burned in kilns at that city. Lime was also produced on a small scale in other parts of the county from 1908 to 1913. No lime production has been reported since that date.
"An extensive deposit of oyster and clam shells in the southern part of the San Francisco Bay has supplied neighboring industrial plants with a low-cost raw material of high lime content for many years. The use of these shells in the production of commercial lime for the magnesia industry was described by Seaton (1942). He stated that higher than normal lime-burning temperatures were required to process the shells, and that the calcining should be done in a basic-lined kiln.
"A plant at Alviso now dredges and processes shells for the poultry industry. (See Mineral Processing.)
Rock - Crushed (in Santa Clara County)
"Substantial quantities of crushed rock have been produced in the county for many years. Most of this material has been used in road construction, although a considerable quantity of fill rock has been required to meet the needs of the post World War II boom in the building construction industry. All rock types, igneous, sedimentary, and metamorphic, are included in the total production."
Serpentine (in Santa Clara County)
"A moderate quantity of serpentine was mined at the quarry of Paul Houret near Almaden during 1946 and 1947. The product was transported to the plant of the Permanente Metals Corporation, Permanente, where it was fused with imported phosphate rock. The resultant calcium magnesium phosphate (Thermophos) was granulated and sold as a fertilizer for soils deficient in phosphorus and magnesium."
Stone (Dimension) (in Santa Clara County)
"During the last decade of the nineteenth century and the first decade of the twentieth century the value of building stone was a substantial portion of the annual value of mineral production in California. As labor costs increased both during and following World War I, the use of stone in new buildings was gradually replaced by less expensive concrete construction.
"Sandstone was first quarried at the Graystone quarries in Santa Clara County in 1866. These quarries are along the southwest flank of the Santa Teresa Hills, about 9 miles south of San Jose on the road to Almaden. The Cretaceous sandstone occurs as hard, buff-colored, massive beds separated by relatively thin beds of clay shale.
"Over half a million dollars worth of this stone was produced between 1900 and 1905 and was used chiefly in the construction of buildings on the campus of Stanford University at Palo Alto. No production has been reported since 1905.
"Tertiary sedimentary beds trending northwestward crop out in the hills along the west side of Los Gatos Creek, south of Los Gatos. Included in this locality are buff-colored beds of Vaqueros sandstone of Miocene age. The sandstone is friable but becomes indurated on exposure to the atmosphere. This stone has been used intermittently for many years by property owners in the construction of walls, wine cellars, and local buildings."
"Considerable lime was burned in Santa Clara County in early days, and the operations have been fully described in the older reports of the State Mining Bureau. The county was among those earliest settled, and easily accessible from San Francisco. The production of lime in the county (not counting that used in the sugar industry) stopped in the early eighties but was resumed in 1908 on a small scale and continued until 1913. Since then, no production has been recorded. Limestone was marketed in small quantities, usually a few thousand tons annually, over a long period until 1940 when production jumped to 190,753 tons, increasing to 280,125 tons in 1941, but dropping to 45,274 tons in 1945. This includes shells as well as hard limestone.
"The hard limestone deposits are in the Franciscan Calera formation (Jurassic ?) which extends southeast along the west side of the county, and is bounded on the southwest by the San Andreas fault. Much of the eastern part of the county is also occupied by Franciscan rocks, but development of limestone has been in the western belt. When carefully sampled and mined selectively, a high-calcium limestone of good grade is obtainable from these deposits. However, the limestone is associated with narrow bands and larger masses of chert which render it undesirably siliceous when mined by methods usually required with such a low priced product. This difficulty was noted in 1889, in a discussion of the Guadalupe Lime Company (Irelan 90),* where it said
(* William Irelan, Jr., Ninth Annual Report of the State Mineralogist for the Year Ending December 1, 1889, California Mining Bureau Report 9, 352 pp., illus., 1890.)
"'One special feature of the lime has been noted and complained of by the parties using the same, that is, the quick setting of the lime when made up as mortar. This is rather a property of cement than of lime, and indicates the presence of silicates in the lime. A close examination of the quarries reveals that the limestone is interstratified with narrow bands of chert often dark colored, and then easily separated by hand, but the limestone itself is often dark or the chert light in color, thus rendering the segregation of the chert well nigh impossible, and it thus finds it way into the kiln.'"
"Eckel (29, p. 357)* alluded to the condition as follows:
(* The #29 indicated in this footnote is not listed in the Bibliography for E. C. Eckel. Perhaps the author meant Report 29: Edwin C. Eckel, "Limestone Deposits of the San Francisco Region," California Division of Mines Report 29, pp. 348-361, illus., 1933.)
"'The entire question of commercial utilization of these Franciscan limestones depends upon this matter of contained chert, not merely upon the amount of silica present in that form, but upon the way in which it is distributed in the limestone beds. For this reason, each individual tract upon which it is proposed to quarry the Calera limestone should require special study and prospecting; it is impossible to take for granted that the conditions found in one set of outcrops will be the same (so far as working conditions are concerned) as those occurring a mile away. There are always beds of high-grade limestone in the Calera formation, but the question always is how much chert must be handled, to produce a ton of high-grade merchantable stone.'
"Since the above were written, floatation has solved the problem beneficiating such limestones. It is now possible to raise the CaCO3 content to 90 percent by this method, and the process can be used on very finely ground stone. Fluorescent (ultra-violet) light can also be used to make hand-sorting from belt conveyors more effective. The chert fluoresces with a different color than the pure limestone, permitting easy separation.
"In December 1939, a portland cement plant began operation at Permanente near Los Altos, Santa Clara County. It is a wet-process plant utilizing the chert-bearing limestone of Black Mountain. Besides making cement, this company has also been producing limestone, especially for sugar refining."