STATE OF CALIFORNIA EARL WARREN, Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM, Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS, Chief SAN FRANCISCO SPECIAL REPORT 3 DECEMBER 1950 COMMERCIAL "BLACK GRANITE" OF SAN DIEGO COUNTY, CALIFORNIA By RICHARD A. HOPPIN and L. A. NORMAN, JR. Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/commercialblackg03hopp COMMERCIAL "BLACK GRANITE" OF SAN DIEGO COUNTY, CALIFORNIA f By Richard A. Hoppin* and L. A. Norman, Jr.** OUTLINE OF REPORT Page granite are locally interrupted by inclusions, spots, and coarse clots. Abstract 3 Some vein-like masses contain minerals of hydrothermal origin which Introduction 3 are so numerous in some places that the rock is unfit for use. Purpose and scope of investigation 3 Two distinct joint sets, a less definite sheeting, and irregularlv Acknowledgments 5 distributed fractures are visible in ledge quarries- several steeply General features of commercial gran.te 5 dil)p ; n g faults are exposed. Rift and grain directions are obscure. Dimension stone industry of San Diego County 5 h ,, t „_._,___ t „ Ko „„,. ii„i ,.„ ,, ; „* , . _■ , ' Geography and general geology of the granite-producing but *™* dr to ., be P a ' all , e to J° ,nt * nd sheeting surfaces. a re a _ 5 The residual boulders have formed chiefly by expansion and Historical sketch 6 subsequent breaking apart of the rock, caused in large part by the Location of quarries and producers 6 oxidation and hydration of the ferrous iron in the ferromagnesian Technology __ 7 minerals and in the iron sulphides. The sulphides weather rapidly, Production and economic conditions 8 so that a monument containing numerous sulphide grains often Utilization and markets 9 becomes badly stained within a few years. Prices __ — _ — .--_ . II rp ne t n j c _ ness of weathered rocks varies greatlv. The amount Geological studies of selected quarries 11 , __••!__■ __ ,..,.* , """""" Introduction _____ 11 of withering is related in part to the distribution and extent of Petrography 13 °^ erosion surfaces. Introduction 13 Objectionable staining of monuments in San Diego cemeteries Composition and texture 13 has been accelerated by sprinkling. White coatings have formed on Imperfections and flaws 14 all types of stones by the evaporation of hard, water and subsequent Hydrothermal alteration 14 deposition of dissolved solids. Sheet structure, joints, and other types of fractures-- - 15 It is recommended that granite producers consider diversifiea- Rtft and grain__ _ 16 tion an(1 stan(,a rdization of products, methods of reducing costs, and Weathering 16 advertising of local stone. Staining of monuments in San Diego cemeteries 17 Conclusions and recommendations 19 INTRODUCTION illustrations P age The utilization of granite and many other natural Figure 1. Index map of San Diego County granite quarries 4 varieties of dimension stone as building materials has 2. Photo of black granite being worked in quarry 8 declined with the advent of less expensive, reinforced I: Phot of SS^^SJSSS^SSJS^A- concrete construction. Granite is still demanded, however, stockpiles for gang saw processing 9 by the monument trade, because the strength, weather 5. Photo of General Petroleum Building, Los Angeles___ 10 resistance and annearanee nf pranitp pan Vip n«Prl pfTW 6. Photo of monument made of San Diego County grano- re__& _______ dnu appearance or granite can oe used ertec- diorite 11 lively; moreover, it conveys a sense of combined beauty, 7. Graphs showing California and San Diego County dignity, and longevity that rarely is derived from other 8. P__rto n of e __OT_menTcarvedfrom San~Diego Coun~ty" materials. The decline in use of granite as a general build- black granite 13 ing stone has been offset some bv its incorporation as an 10: FEoto ol SS^SS^'^^^I^i: S architectural or veneer stone in present-day buildings to 11. Photo showing sheet structure 15 provide an accent of beauty and quality. Greater and 12. Photo^jE partly weathered rock traversed by numerous ^ more effective use of granite seems possible in dwellings, 13. Photo of Came7o~nbVeHng"quarryrLakesi~d~e~IZZIZIZI 16 commercial and public buildings, and in structures that 14. Photo of perfect joint plane _ — 17 are a part of the vast California highway development 15 " ^S^ 9 ^-^^—-.^^-^^-- 17 program now under way. 16. Sketch of McGee quarry, Pala 18 The granite industry of San Diego County, never 17. Photo showing disfiguration of black granite v j in volume or va l U e, reached its greatest peak monument i« __._••__•■ • ■ 18. Photo showing disfiguration accentuated on carved ot productivity during recent years, a Situation not true surface of monument 19 f the industry in California as a whole. Both the county abstract anc * state industries are confronted with the problems of retaining their present markets and entering new ones. Granite for monumental and building stone use has been Satisfactory solution of these problems will depend not quarried for many years in the western half of San Diego County. on , upon "general economic conditions, but to a great principally in the Lakeside, Escondido, and San Marcos-Vista areas. , , _i a? i? ..i i • i ?• , The output of stone during the 51-year period 1898-1948 was 577,276 extent u P on the efforts of the Producers to provide high- cubic feet valued at .$1,344,273, which represents 5.8 percent of the quality stone at reasonable and competitive prices, and total California production for that period. In 1948, the county con- to pursue vigorous, well-organized campaigns for wider tributed about 30 percent of the total granite production of the state. acceptance of their products by the building, construe- The commercial granite quarries of San Diego Countv are in ,. ■, , . -, , . gabbroic ("black granite") and granodioritic rocks of a large com- tl0n > anCl monument industries, plex batholith that underlies most of the Peninsular Range province. Purpose and Scope of Investigation Ihis batholith has intruded Mesozoic schists, quartzites, and volcanic rocks along the western border of the province, and Paleozoic meta- The present studies of dimension stone in San DiegO sedimentary rocks along the eastern border. Most black granite Countv were initiated bv a request from the San DietjO quarries have been operated in residual boulder deposits; light-gray tv v •'„ »c vr-* „i t> i. _v. oix _ t\- ■ ■ _ granite has been quarried mainly from massive ledges. P;™ 10 " ° f Na . tural .^sources to the State Division of The black granite varies in composition.and includes such petro- Mines for an investigation of certain disfigurations of graphic varieties as hornblende gabbro, norite, and quartz-biotite cemetery monuments made from "black granite" quar- gabhro. Th e homogeneous color and texture characteristic of the black r i e d in the county. The problem, similar to that encoun- t Manuscript .submitted i for publication December 22, 1949. tered in the cemeteries of many communities, involved • California Institute of Technology. . , . „ , J . . ' •* Senior mining geologist, California Division of Mines. not Only Studies Ot the monument Staining, but alSO led (3) Special Report 3 Figure 1. Index map of San Diego County granite quarries (monumental and building stone). Commercial "Black Granite" of San Diego County r. to a study of the stone and its sources. Presented in this report are the results of these investigations, which have included reconnaissance field study of several typical dimension-stone quarries in the county, together with related laboratory work on specimens from these quarries. A brief historical and economic sketch of the county's granite industry is also included. The quarry studies were focused upon the composi- tion and texture of the rocks; the nature and extent of natural weathering ; the occurrence of joints, fractures, faults, sheeting, and other structural features ; and upon the nature and distribution of hydrothermal alteration within the rocks. The field study was supplemented by laboratory examination of nearly one hundred thin sec- tions of fresh, weathered, and hydrothermally altered rock. An attempt was made to correlate the field and laboratory studies with problems of quarrying and com- mercial exploration in general. The historical and economic data contained in the report have been obtained from published and unpub- lished reports of the Division of Mines, other published reports, and from data collected in the field. Richard A. Hoppin is responsible for the sections of this report that deal with general features of commercial granite, geography and general geology of the granite producing area, geological studies of selected quarries and staining of monuments in San Diego cemeteries. The introduction and sections dealing with historical aspects, location of quarries and producers, technology, produc- tion and economic conditions, utilization, and prices are the work of L. A. Norman, Jr. The conclusions and rec- ommendations are the joint responsibility of both authors. Acknowledgments All investigations were carried on in close coopera- tion with the San Diego County Division of Natural Resources. Roy M. Kepner, Jr., Assistant Director, par- ticipated in several field conferences and aided in the com- pilation of background data. The authors also are indebted to Richard H. Jahns, California Institute of Technology, for his suggestions in carrying out the studies and for his critical readings of the manuscript. Ralph E. Blake, Pyra- mid Granite Company, Escondido, generously contrib- uted much of his time. The index map and sketch of the Pala quarry were drafted by Ellen Powelson, California Institute of Technology. Sincere appreciation is extended to present and former granite producers, quarrymen, and operators of finishing shops for their cooperation in supplying infor- mation and providing access to quarries and plants, as well as for their many personal kindnesses. GENERAL FEATURES OF COMMERCIAL GRANITE 1 The term "granite" is used in this report in the general commercial sense, and includes rocks that range in composition from gabbro to true granite. Geologically, granite can be defined as an igneous rock in which all the crystals can be distinguished individually by the naked eye, and which consists of quartz ; potash feldspar, subor- dinate sodic plagioclase feldspar, and generally mica or 1 Bowles, Oliver, The stone industries, McGraw-Hill Book Com- pany, 2d ed., 519 pp., 1939. Ries, H., Economic geology, McGraw-Hill Book Company, pp. 102-130, 1937. hornblende. Certain closely related igneous types such as syenite, diorite, tonalite, monzonite, and granodiorite, are grouped with granite in most commercial classifica- tions. They can be distinguished from true granite only by careful study. The "black granite," or gabbro, of San Diego County is similar to true granite in structure, texture, and commercial use, but differs significantly in color and composition. It is composed essentially of pla- gioclase feldspar, several varieties of pyroxene, horn- blende, and biotite, and contains little or no quartz. The adaptability of granite for structural or orna- mental use is governed chiefly by such physical properties as texture, color, and hardness, and by such structural features as joints, sheeting, fractures, rift, grain, dikes, spots, inclusions, and hairlines. Uniformity of color and texture are desirable, and for most uses the stone should be capable of taking a smooth, long-lasting polish. Al- though all granites are hard, they vary considerably in brittleness and toughness, depending in part upon the degree to which the mineral grains are interlocked. The number, arrangement, and spacing of joints are important in quarrying. Two straight major joint sets that intersect at right angles are advantageous, particu- larly where the fractures lie 10 to 30 feet apart. The pres- ence of rift and grain aids immeasurably in splitting the stone. These features are represented by planes along which the rock may be split with relative ease. The rift is the direction of easiest splitting, and the grain, which normally is perpendicular to the rift, is a direction of splitting only slightly less pronounced. Dikes, spots, inclusions, and hairlines mar the ap- pearance of the stone and add to the cost of the operation. The dikes, which are tabular bodies of igneous rock that cut across the structure of the enclosing rock, vary in width from a fraction of an inch to several feet. Spots are usually due to segregations of light or dark minerals and are about the size of a match head. Inclusions are irregular masses somewhat larger than spots and com- monly consist of material that differs in color, compo- sition, and texture from the enclosing rock. Hairlines are fine lines of discoloration caused by alteration along joints and fractures, or by the presence of thin mineral veinlets. Granite is strong and durable. The crushing strength of commercial granite ranges from 15,000 to 30,000 pounds per square inch. Granite will stand exposure to weather for many years before showing signs of decay. This period of time varies for a given stone, depending upon the location and the climate ; for most granites it ranges from 75 to 200 years. 2 DIMENSION STONE INDUSTRY OF SAN DIEGO COUNTY Geography and General Geology of the Granite-producing Areas The granite quarries of San Diego County lie within the Peninsular Range province. The Peninsular ,Range province includes those parts of Baja California and Southern California that lie south of the San Gabriel and San Bernardino Mountains, west of the Salton Imperial depression, and east of the Pacific coastal plain. Most of the province is hilly or mountainous, and many of the » Ries, H., op. clt., pp. 107-108, 1937. (i Special Report 3 individual mountain masses along the eastern boundary of the province are 5,000 to 10,000 feet high. In the lower granite-producing areas farther west, the general relief is approximately 1,500 feet. Except where recently burned over, the hills are covered with a thick growth of brush. Where underlain by gabbroic rocks, or "black granite," these hills are mantled with reddish-brown soil. Many of the more silicic rocks, in contrast, form rough, boulder-strewn ridges and slopes. The rainfall in the lower, westerly parts of the province averages about 15 inches, and occurs mainly dur- ing the period December-April. The winters are mild and the summers characteristically hot and dry; the climate ordinarily permits operation of the quarries on a year- round basis. A large part of the Peninsular Range country is underlain by intrusive igneous rocks of probable Upper Cretaceous age. 3 These rocks represent a great, complex batholith, and were intruded into Mesozoic schists, quartz- ites, and volcanic rocks along the western border, and into Paleozoic metasedimentary rocks along the eastern border of the province. Like the province as a whole, the batholith is elongated in a southeasterly direction. The intrusive rocks are exposed continuously from Riverside, Califor- nia, to points many miles south of the Mexican border, a distance of more than 100 miles within the state of Cali- fornia. The average width of the batholith is about 60 miles. Its western margin is in part overlapped by younger sedimentary rocks. The "black granite" quarries in San Diego County are in large intrusive masses of San Marcos gabbro, which is the oldest of the major rock types in the batholith. 4 This gabbro, or black granite, forms numerous stock-like bodies that in many places are entirely or partly sur- rounded by larger, younger masses of tonalite and grano- diorite. The largest exposure of black granite in the area extends over about 11 square miles ; others cover 5 square miles or less. Historical Sketch The granite industry in San Diego County evolved from local use of the stone by early settlers. Recorded production of granite as a dimension stone dates from 1898, although granite quarries were operated several years earlier. Large tonnages of rubble and riprap were produced for breakwater and dam construction during the years near the turn of the century. This bulk use of granite has continued to the present, but during recent years has become distinct from dimension stone uses. Production of paving blocks cut from granite was first reported in 1910, and during succeeding years a small but flourishing industry was supported by the demands of railroads for rail-lining material in urban areas. Profi- cient stonecutters, many of them trained in the European countries of their birth, produced blocks by hand methods at numerous quarries in the Grossmont, Santee, and Foster areas. Some of these old quarries are visible in the low hills half a mile south of Santee and along the west side of the Lakeside-Ramona highway, a few miles north * Larsen, E. S., Jr., Batholith and associated rooks of Corona, Klsinore and San Luis Rey quadrangles, southern California : Geo). Soc. America Mem. 29, 182 pp., 1948. 4 Miller, P. S., Petrology of the San Marcos gabbro, southern California: Geol. Soc. Am. Bull., vol. 48, pp. 1397-1426, 1937. of Lakeside. An important factor in these quarrying activ- ities was the proximity of the San Diego, Cuyamaca and Eastern Railway, now abandoned. Dimension stone for building and memorial purposes found a gradually increasing market, partly as a result of wider recognition of the quality of the stone, and partly because of sales efforts by the quarrymen, who were in search of a new market to replace declining demands for riprap and paving stone. James Simpson of the Simpson- Pirnie Granite Company, San Diego, was one of the pio- neers in the stone business, operating quarries at Santee and Foster from 1888 to 1932. The company reported sales of building stone, monumental stone, paving blocks, rubble and riprap. This diversified production probably accounts in large part for their 46-year period of opera- tion, longest of any in the county. Dimension stone for use in the construction of government buildings at Fort Rosecrans, on Point Loma near San Diego, was quarried in the Foster area by the Waterman Granite Company in 1903-04. 5 All of this early work was done in a region of pre- dominately light-gray granodioritic rock ; it was not until 1921 that the currently important black granite (gabbro) was exploited extensively, although Jose Cova produced small amounts of the stone before 1921 from a quarry near Santee. In 1921 the Bly Stone Company, Los Angeles, and W. E. Van Deventer reported sales of such material from two quarries near Bernardo. In 1922 Robert J. Magee reported black granite production from his quarry northeast of Pala, in the northern part of the county. About the same time, the area southwest of Escondido was opened up ; it is now a major source of stone. First production from the San Marcos- Vista area was in 1938. John Stridsburg, present operator of the Crystal Black granite quarry, has been active in the Escondido area since 1923, and has reported continuous production under his own name since 1926. All quarries and producers listed in the records of the State Division of Mines are shown in table 1. Many other companies and individuals, including those failing to submit production records, those producing granite products other than monumental and building stone, and the many stone workers, are not listed but nevertheless played an important part in the development of the county's granite industry. Location of Quarries and Producers The granite quarries of San Diego county are shown in figure 1. There are three principal centers of produc- tion, all in the western half of the county. Lightf-gray granodiorite is obtained from an area within a 5-mile radius of Lakeside, whereas most of the black granite is obtained from two areas, one within a 5-mile radius of Escondido and the other within a 3-mile strip that lies 4 miles east of Vista. The confinement of quarrying activ- ity to these areas probably is due partly to their p'osition with respect to transportation, as well as to the wise tendency to continue operations in a known area of good stone. Undoubtedly there are other deposits of satisfac- tory stone that could be exploited if the demand were sufficient. 5 Aubury, Lewis E., The structural and industrial materials of California : California Min. Bur. Bull. 38, p. 52, 1906. Commercial "Black Granite" of San Diego County Table 1. San Diego County granite producers of building and monumental atone * Operator American Marble and Granite Works Atcheson, Topeka and Santa Fe Railway Bly Stone Company California Cut Stone and Granite Company . Cameron and Deering_ Clemens Granite Company C^vas. Jose Daley Corporation.. Deering and McDonald Ebony Black Diamond Granite Company. .. Fellows and Clutter Grant, John Johnson, Emil and Sons (National Quarries) Johnson, Emil and Sons (National Quarries) Lehnberg, Andrew Lodge, Alvin I McCarthy, Daniel _. McGilvray, Raymond Corporation McKoon Granite Quarry Magee Quarry (Robert J. Magee) Mat son, Pete Matson and Deering Matson and Deering Matson and Deering — Matson and Kouns Matson and McDonald Meyers Granite Quarry (W. A. Meyer) Mission Silver Gray Granite Company Moore, Charles G Pacific Cut Stone and Granite Company Pomona Granite Company Riverside Monumental Works San Diego Granite Works Simpson-Pirnee Granite Company Simpson-Pirnee Granite Company Southern California Granite Company Stockdale Granite Quarry (F. W. Stockdale) (See Ebony Black Diamond Granite Company) Stridsburg, John Texas Quarries Valley Granite Company Van Deventer, W. E Waterman Granite Company. Index map number 10 District San tee Lakeside-Foster Bernardo Vista Lakeside-Foster Suncrest Santee Bernardo Lakeside-Foster Escondido Vista Grossmont, Santee, Lakeside Foster San Marcos Pala Foster Lakeside-Foster Suncrest Lakeside- Foster Lakeside- Foster Pala San Marcos Lakeside-Foster Lakeside- Foster Lakeside-Foster San Marcos.. San Marcos Lakeside- Foster Lakeside-Foster Grossmont- La Mesa Escondido San Marcos Pala Grossmont Santee Lakeside-Foster Lakeside-Foster Escondido Escondido San Marcos Escondido Bernardo Foster Color of polished stone Black Gray . . Black. Black Gray . . Gray.. Black . Black . Gray.. Black Black . Gray. Black . .. Black . Gray Gray Gray Gray Gray Black . . . Black . . Gray Gray Gray Black... Black... Gray Gray Gray Black . . . Black... Black ... Gray Gray Gray Gray Black... Black.. Black... Black . . Black... Gray Trade name Blue-Diamond granite. Vista Black granite Lakeside Silver Gray granite. Suncrest Gray granite San Diego Black granite Mission Silver Gray granite. Ebony Black granite National Blue granite. Pala Black granite Mission Silver Gray granite. Suncrest Gray granite Silver Gray granite. California Black granite Mission Silver Gray granite. Mission Silver Gray granite. California Black granite. California Black granite. First year of production 1927 1913 1,921 1945 1945 1946 1900 1923 Mission Silver Gray granite. "Black Diamond Granite" 2 . Mission Black granite Silver Gray granite. Superior Black granite (former) Crystal Black granite (present) Escondido Black granite. 1923 1945 1913 1946 1946 1912 (?) 1945 1945 1921 1924 1922 1948 1932 1945 1932 1944 1938 1923 1929 1908 (?) 1936 1945 1945 1912 (?) 1888 (?) 1919 1924 1926 1944 1944 1921 1903 Last year of production 1941 1913 1924 1947 1946 '1948 1926 1923 1936 1946 1915 '1948 '1948 (?) 1946 1945 1935 1929 1934 '1948 1936 1946 1936 1946 1944 1930 1932 (?) '1948 1945 1945 (?) 1932 1932 '1936 1924 '1948 1944 '1948 1925 1904 * Compiled from production records and reports of the California Division of Mines and from data obtained in the field. ' Producing In 1949. 5 Registered and copyrighted, U. S. Patent Office. The list of producers in table 1 shows clearly the high mortality rate of producers in terms of length of opera- tions. Of the seven 1948 producers, only two reported pro- duction prior to 1937. Also of interest are the numerous trade names applied to the stone that has been marketed. One of them, "Black Diamond Granite", is registered and copyrighted with the U. S. Patent Office. The 16 dif- ferent names noted in table 1 form only a part of the com- plete list, no accurate record of which is available. Technology Hardness, texture, and other properties that sub- stantially affect the suitability of granite as a dimension stone also creates many problems in quarrying and prepa- ration. Techniques developed for quarrying softer types of dimension stone are not always applicable to operations in granitic rocks, where production costs are often rela- tively high. Quarrying and finishing methods have been well described by Bowles 6 in his comprehensive study of the stone industries. The granite quarries of San Diego County are small to moderate in size, and are of two distinct types. In gen- • Bowles, Oliver, op. clt, pp. 143-167, 1939. eral, the black granite quarries have been in deposits of residual boulders, whereas most granodiorite quarries have been operated in ledge, or massive rock. Both types of quarries are commonly started as shelf excavations, with the development of an opening horizontally into the hillside. Black granite quarries ordinarily have been con- tinued as "shelf quarries," but some of them and most of the granodiorite quarries have been deepened into "pit quarries, ' ' in which the rock is obtained from points below the original quarry floor. After exposure in the quarries, boulders are split by drilling and blasting. The split portions of the boulders then are worked into smaller blocks by the "plug and feather" method (fig. 2). In granodiorite quarries, maxi- mum advantage is taken of the joint sets to obtain large sized quarry blocks from which smaller ones are made. Equipment used in the quarries consists of the usual stoneworkers ' hand tools, pneumatic hand drills, portable compressors, single derricks, and in sonie places, a bull- dozer to move waste material. Only one quarry is equipped with an electrically-powered steel derrick and gang saw. The saw is used to produce slabs of various thicknesses (figs. 3 and 4). Special Report 3 ■"-■sp *vl&k 1 *'ft' ' "" jjf ^M iKift'* '■-/ ■; 9*4 1* ■rr- "~* ^Sl •. 4| k -^ s ^ 1^*3 1 ^^H Figure 2. Boulder of black granite being worked in quarry. Johnson and Sons quarry, Vista. Emit Disposition of waste in quarrying operations is al- ways an important problem. Operators of quarries in black granite must handle both waste rock and the disin- tegrated material that surrounds the boulders. The latter is used to maintain quarry floors, and at one place is also sold as surfacing material for driveways and roads. Many of the older granodiorite quarries, as judged from present appearance, were literally "worked into a hole," with poorly planned development of steep or overhanging walls on one or more sides, excavations of awkward shape, and with rapid accumulation of waste for which satisfactory disposition was not provided. The quarrying methods typically used in San Diego County appear crude and inefficient as compared with those of other areas, particularly the important granite centers of the eastern United States. The local quarries, however, present difficult and unusual problems. Many excellent techniques ordinarily used in the quarrying of massive rock are not feasible for residual boulder deposits. All known techniques should be studied and considered, however, as many of these might be useful, and might lead to reduction of operating costs. The physical characteristics of granite contribute to problems of finishing, as well as quarrying the stone. No finishing shops are located on quarry premises in the county, although two of the producers active in 1948 maintain such shops elsewhere. These and other finishing shops in the county use essentially standard methods and equipment as described by Bowles. 7 Location of shops at or near quarries, adoption of improved methods and equipment, and attention to the possibilities of produc- tion-line methods might contribute to lower costs. Production and Economic Conditions Production of building and monumental granite in San Diego County during the 51-year period 1898-1948 has amounted to 577,276 cubic feet valued at $1,344,273, or 5.8 percent of the total California production for that period. In 1948 the county production constituted about 30 percent of the total state production. The output of the county's industry has fluctuated greatly, with trends 7 Bowles, Oliver, op. cit., pp. 156-167, 1939. generally parallel with those of the state's industry. Cali- fornia's greatest output, in terms of value, was reached in the years 1921-30, an active period for the San Diego County industry as well. This was due mainly to the ac- celerated building activity of the period. The record year in state production, 1925, was second highest for the county, whose maximum output was reached in 1947. As shown in tables 1 and 2, general business condi- tions are reflected in the granite production figures for both California and San Diego County. Financial depres- sions, labor troubles, building booms, and periods of pros- perity have shown their effects during the years. 8 The panic of 1907, for example, had little effect upon state production, but a marked effect upon county production, which dropped sharply in 1908. The financial depression before World War I, in contrast, resulted in a lowering of both state and county production. Although the county production improved in 1915 over the 1914 low, the state production did not owing in part to a strike of stone- cutters. 9 FiGURE 3. Gang saw with partly completed cuts in black gran- ite boulder. Pacific Cut Stone and Granite Company quarry, Escondido. The period 1916-18, with its labor troubles, short labor supply, higher operating costs, and other wartime conditions, was a serious one for both the national and the California stone industry. The state's 1918 production of $139,861 was its lowest since 1888. County production during the same period was at a low level. The years 1921 and 1922, in which many post-war economic adjustments 8 Bowles, Oliver, and Hatmaker, Paul, Trends in the production and uses of granite as dimension stone : U. S. Bur. Mines Rept. of Inv. 3065, 1935. Data on national economic conditions, as related to the granite industry, are well summarized in this paper, and have been used by the authors for comparison with California and San Diego County production. "Bradley, Walter W., California mineral production for 1915- California Min. Bur. Bull. 71, p. 58, 1916. Commercial "Black Granite" of San Diego County were made, marked the start of California 's most im- portant years of production, and likewise a correspond- ing period of prosperity for the industry in San Diego County. Increased building activity, particularly the con- struction of public buildings in the Los Angeles area, was directly responsible for the high output of dimension granite in 1924-25. 10 The depression of the early 1930 's is clearly reflected in the decreased production figures for those years. Full recovery of the industry a few years later was retarded by World War II, but the subsequent 5-year period of improvement, dating from 1944, saw peak production in San Diego Countv. Table 2. Production of building nnd monumental (inutile 1898-1948 ' m m Figupe 4. View of quarry with residual boulders in place and stock- piles for gang saw processing. Sawed slabs in foreground. Pacific Cut Stone and Granite Company quarry, Escondido. Utilization and Markets Granite quarried from deposits in San Diego County has been used for a variety of purposes, but the greatest amount in terms of production value, has gone into memorials. This usage, an accurate record of which is not available, follows that of the national consumption of granite. 11 Monumental use, which normally requires that a stone take a smooth polish, demands rock of the highest quality, as texture, color, and physical defects are easily seen in the polished stone. Both granodiorite and black granite are in constant demand for monumental purposes. The popularity of black granite is indicated by production figures from 1921, when the rock was first extensively quarried. Prom 1921 to 1948 the output of this rock constituted 51 percent of the total production of dimension stone in the county. For the entire 51-year period, 1898-1948, it constituted 43 per- cent of the total, and for the last five years, 1944-48, it amounted to about 90 percent. Monuments made from this stone are sold not only throughout California, but in many other states, Canada, Mexico, and the Hawaiian Islands. The recently completed Flagpole Memorial, at the Nevada entrance to Hoover Dam, is made from the black 10 Bradley, Walter W., California mineral production for 1925: California Min. Bur. Bull. 97, p. 70, 1926. 11 Bowles, Oliver, op. cit., p. Ill, 1939. Year 1898 1899 1900 1901 :ao2 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 Total California Value (in dollars) 147,732 141,070 295.772 519,285 255,239 678,670 467.472 353,837 344,083 373,376 512,923 376,834 417,898 355,742 362,975 981,277 628,786 227,928 535,339 221,997 139,861 220,743 495,732 725.901 676,643 760,081 211,046 ,853,859 655,332 398,443 763,996 169.271 855,477 636.741 398,676 183,706 249,083 339,917 244.243 207,738 131,386 145,194 198,896 261,661 186,872 148.160 222.843 220,411 275,367 305,303 285.214 $23,766,031 San Diego County Cu. ft 6.588 11.000 7.300 13,900 20.590 10,800 7,760 9,500 9,500 19.900 9.000 2,200 18.311 5,425 6,900 6.900 5,500 5,600 6.690 2 8,499 15.931 19,715 26,809 28,506 16,273 18,858 14,239 15,835 9,986 4,704 6,232 5.342 6,333 6,189 14.065 10,863 6,030 7,183 12,638 28,257 29,489 31,062 25,263 577,276 Value (in dollars) 4,875 8,150 9.900 22,400 13.175 13,548 7,851 10,250 10,250 23,650 10,000 2,200 21,708 6,425 8.300 8,300 6,150 9,200 15,215 2 22,444 35,673 40,600 76,532 108,703 45,327 63,142 47,572 28,864 27,411 11,402 9.700 10,097 11,167 10,614 28,000 29.017 14,233 15,391 43,922 85,892 92,791 115,135 94,958 $1,344,273 Unit value per cubic foot (in dollars) 0.74 (est) 0.74 1.36 1.61 0.64 1.25 1.01 1.08 1.08 1.19 1.11 1.00 1.19 1.18 1.20 1.20 1.11 1.64 2.27 2.64 2.24 2.05 2.85 3.81 2.79 3.35 3.34 1.82 2.74 2.42 1.56 1.89 1.76 1.71 1.99 2.67 2.36 2.14 3.48 3.04 3.11 3.71 3.76 1 Tabulated from records of the Califonia Division of Mines. Certain figures not i" agreement with published records represent inclusion of returns not received in time for use in earlier reports or reclassification of returns. - Omitted to conceal output of individual producers; included in total. granite. As a building stone, this rock is used extensively in veneers and facings, alone or incorporated with other natural stone, or with manufactured and synthetic build- ing materials. Its dark color and uniform texture provide a pleasing accent, and harmonize well with other mate- terials. Moreover, the stone is easily maintained after installation. The recently completed General Petroleum Building in Los Angeles (fig. 5) illustrates the use of San Diego black granite in combination with terra cotta. A few of the many other buildings in which this stone has been used are the Mirror Building, Western Union Building, Broadway Crenshaw, Saks 5th Avenue, and Bullock's 10 Special Report 3 B g ■5 o s ^ o t- £ eij B a ** o oO o » 3» 3 8 O o si CO 8 .A VI CD ■J Commercial "Black Granite" op San Diego County 11 Pasadena in the Los Angeles area, and the Appraisers Building and Hastings Building in San Francisco. Its use in the Minnesota Mining and Manufacturing Com- pany Building, St. Paul, Minnesota, Salvation Army Building, Detroit, Michigan, and in the United States Embassy Building, Havana, Cuba, reflects its widespread distribution outside the state. Light-gray granite of the county, used so extensively in earlier years for construction purposes, also has found great favor as a monumental stone. Its even texture and "silver-gray" color make it suitable for many purposes. Unfortunately, production of this rock has dropped to a negligible amount during recent years, and its once well- established position is threatened by lack of supply. Both producers and finishing shops have indicated that there has not been sufficient available stock to fill recent demands. Expansion of markets for San Diego dimension stone will require considerable effort on the part of operators to produce quality stone at lowest possible cost, and to wage a campaign for further acceptance of their prod- uct. Diversification of sales should be promoted ; some materials, such as small blocks for ornamental and build- ing purposes, might well be marketed from present waste. Such stone is used, for example, in San Diego County park installations. Small pieces also could be used, alone or with other building and decorative stones, in walls, highway structures, curbings, and walks. Crushed rock is another possible outlet for present waste. The produc- tion of a wider variety of products is dependent, of course, upon opportunity to market them economically and com- petitively with other materials. The potential gain from such diversification seems great enough to warrant a care- ful study of the problem. Prices Average annual prices of San Diego County building and monumental granite, have ranged from 64 cents to $3.81 per cubic foot. The average unit price during the period 1898-1910 was $1.07 per cubic foot, as compared with $1.55 in 1911-20, $2.76 in 1921-30, $2.12 in 1931-40 and $2.99 in 1941-48. Years of highest unit value also have been years of higher output in total value and volume. The unit price of $3.81 in 1925 was the highest in 51 years, but the year ranks second in value and third in volume ; 1948 ($3.76) ranks third in value and sixth in volume; 1947 ($3.71) ranks first in both value and volume. High unit prices for rough quarry stone have been accompanied by high prices for finished products. Bowles and Hatmaker 12 raise the question as to whether "the rapidly increasing prices (of memorials) may not have retarded sales and abetted a trend toward wider use of community mausoleums," thus adversely affecting over- all consumption. High prices also result in use of smaller stones, cheaper substitutes, and, in many instances, post- ponement of purchases. Prices, dependent upon produc- tion and finishing costs, must be competitive and must be aligned with purchasing power if the industry is to flourish. GEOLOGICAL STUDIES OF SELECTED QUARRIES Introduction Seven operating quarries were studied during the spring of 1948. Six of these quarries have been developed in the San Marcos gabbro, or black granite, and the sev- enth is in granodiorite. The quarries examined are the John Stridsburg, Pacific Cut Stone and Granite Com- pany, and the Valley Granite Company near Escondido; Pete Matson at Vista ; Emil Johnson and Sons at Pala and at Vista ; and the Cameron-Deering at Lakeside. The loca- tion of these quarries is shown on the index map (fig. 1). ' 2 Bowles, Oliver, and Hatmaker, Paul, op. cit., p. 6, 1935. Figure 6. Monument made of San Diego County granodiorite. Residual boulders are being worked in all the black granite quarries except the Stridsburg. These boulders are remnants of fresh rock surrounded by crumbly, decom- posed material. The massive granite is traversed by a net- work of joint, fracture, and sheeting surfaces that divides the rock into blocks of various sizes. The water descending through the narrow openings along these surfaces con- tains a small amount of carbonic and sulphuric acids. Because the minerals of the granite crystallized at higher temperatures than exist in this surface environment, they are susceptible to alteration by these cold, dilute acidic solutions. Decomposition and disintegration take place, moving inward from all sides of each fracture-bounded block and gradually rounding its corners until only a boulder is left. This results in a concentric layering that has been called onionskin structure (fig. 9). The remain- ing boulders range from a few inches to 30 feet in diameter. The quarry at Pala (fig. 10) exposes some interesting relations between fresh and altered rock. The material between the boulders shows none of the onionskin struc- ture that is typical of the residual rock' in the other quar- ries. Instead, a close inspection of this material reveals broken grains of feldspar and other fragmental features. The contact between the boulders and the matrix is sharp. It seems significant that the quarrymen have been unable 12 Special Report 3 < _l o Q U- O 1900 1910 1920 1930 CALIFORNIA GRANITE PRODUCTION Dollar Value 1898-1948 1940 1950 100 < — i _i o a tn o z < o 50 A A*A /U/ / 1900 1910 1920 1930 1940 SAN DIEGO COUNTY GRANITE PRODUCTION Dimension Stone- Dollar Value 1898-1948 1950 & 3.00 o an 3 a 2 150 _■ o a 1900 1910 1920 1930 1940 1950 SAN DIEGO COUNTY GRANITE PRODUCTION Dimension Stone -Unit Value and Volume, 1898-1948 Figure 7. Graphs showing California and San Diego County granite production. Curves are broken to conceal production of single producers for period. Commercial "Black Granite" op San Diego County 13 to locate a consistent rift direction in these boulders, whereas in the other quarries such directions have been found in even the smallest boulders surrounded by large volumes of weathered rock. It is evident that the near- surface boulders at the Pala quarry have been rotated and moved, presumably by mass slumping and creeping. However, residual material is now being uncovered near the bottom of the quarry, where onionskin structure and other features characteristic of such masses are clearly visible. An undulatory boundary between the "creep" material, with its rotated boulders, and the underlying residual material was well exposed in 1947 (fig. 16), but has been since concealed by talus and slump debris. Petrography Introduction A complete account of the geology of the batholith of southern California was recently published by Larsen. 13 Miller has made a thorough study of the petrography and petrology of the San Marcos gabbro. 14 The details of the petrography therefore are not repeated here ; instead, the petrographic features are discussed in the light of their influence upon the use of the rock as a monumental stone. The commercial designation ' ' black granite ' ' is used here for the gabbroic rocks. More specific petrographic names are employed if a specific rock type is discussed. Typical mineral assemblages of the fresh rock are given in table 3. Each of these is an average composition determined from seven thin sections of specimens taken from each quarry named. Figure 8. Monument carved from San Diego County black granite (gabbro). Photo by Ken Franklin, Jr., courtesy of San Diego Division of Natural Resources. Composition and Texture Gabbro. The gabbro, or black granite, is dark gray to black in color, and the percentage of light and dark components is fairly constant. The minerals are rather 13 Larsen, E. S„ Jr., op. cit., 1948. « Miller, F. S., op. cit., pp. 1397-1426, 1937. Miller, F. S., Hornblendes and primary structures of the San Marcos gabbro: Geol. Soc. Am. Bull., vol. 49, pp. 1213-1232, 1938. evenly distributed, so that the rock has a homogeneous appearance. The relative amounts of the mafic minerals vary considerably throughout the black granite, and vari- ations of as much as five percent are common within a single quarry. These variations appear to be a function of the degree to which reaction took place during a late stage in development of the rock, between the residual liquid and the materials already crystallized from the original magma. Hypersthene formed first and was fol- lowed by augite. Both pyroxenes were rimmed by a dark green hornblende, and locally were almost completely replaced. With further reaction, biotite was developed, together with some fine-grained interstitial quartz. Along with these changes, the plagioclase became increasingly more sodic and zoned crystals were formed. With still more reaction, a rock with composition near that of a tonalite resulted (note rock no. 13, table 3). Table 8. Estimated mineral percentages of fresh rock. Quartz Potash feldspar Plagioclase- Hornblende Hypersthene Augite Biotite Iron ore % An in plagioclase. Sample number 37 30 30 3 Tr 20 19a 15 Tr 68 24 Tr 3 Tr 5 55 60 35 Tr 3 Tr 2 55 21 65 10 14 10 0.5 0.5 55 1(1 Tr 64 15 10 10 Tr 1 60 13 10 55 8 1 6 19 1 45-55 Sample 4. Leucogranodiorite: Cameron-Deering, Lakeside Sample 19a. Hornblende gabbro: John Stridsburg, Escondido Sample 15. Hornblende gabbro: Pacific Cut Stone and Granite Company, Escondido Sample 21. Hornblende norite: Valley Granite Company, Escondido Sample 10. Hornblende norite: Emil Johnson and Sons, Pala Sample 9. Norite (with hornblende, quartz, biotite) : Emil Johnson and Sons, Vista Sample 13. Quartz biotite gabbro: Pete Matson, Vista Iron sulphides amount to only a few tenths of one percent of the rock. Nevertheless, when the rock is exposed to weathering, they are the most deleterious group of minerals present, because they oxidize and form stains. Pyrite is by far the most common, but where sulphides are relatively abundant, scattered grains of pyrrhotite and chalcopyrite are also present. These minerals, which are best observed on polished faces in bright reflected light, weather rapidly on exposure to the atmosphere. They form brown iron oxide, so that a monu- ment containing numerous grains of sulphide minerals becomes badly stained within a few years or tens of years. There appears to be a direct correlation between the amount of sulphide in the rock and the amount of ,horn- blende — especially a pale blue variety of hornblende. Considerable magnetite is present in some of the rocks, but because it oxidizes very slowly on exposure to weathering, it is not important in the formation of brown stains. Some of the magnetite was formed from the iron oxide released as a result of the replacement of hypersthene and augite by hornblende, and some is asso- ciated with the biotite of later reactions. Part of it prob- ably was introduced with the sulphides, and is in close association with them. The black granites are medium-grained, the constitu- ent mineral grains averaging 1.55 mm. in diameter. Most grains are irregular in shape, though some of the feldspars 14 Special Report 3 have fairly well defined crystal forms. The texture is gen- erally seriate, the mineral components ranging in size from a fraction of a millimeter to more than 3 mm. long. In a few localities, however, the rocks have a porphyritic texture. Some of the large feldspar grains are as much as 4 mm. long. No preferred orientation was observed. The »rain size has a marked effect upon the color of the rock. The coarse-grained black granites are lighter in color than those of finer grain. For example, the quartz- biotite gabbro (no. 13, table 3) is dark gray on a polished surface, whereas the hornblende gabbro (no. 10, table 3) is black where polished even though the quartz-biotite gab- bro has a higher proportion of dark-colored minerals. Granodioritc. The rock near Lakeside is a fine- grained, light-gray leucogranodiorite. The prefix "leuco" is applied because the rock contains only a small amount of dark constituents (no. 4, table 3). The feldspars and quartz are about 0.5 mm. in average diameter, the biotite about 0.3 mm. A few scattered phenocrysts of oligoclase are as much as 3 mm. long. They are markedly zoned, and their more calcic cores are selectively altered to sericite and calcite. These grains are conspicuous on most polished surfaces, because of the slightly yellowish color imparted to them by the alteration. No sulphide minerals are present. Figure 9. Residual boulders in granular matrix. Emil Johnson and Sons quarry, Vista. Imperfections and Flaws The homogeneity of color, grain, and composition that is characteristic of the black granite is interrupted to some degree by several types of imperfections. Most of these flaws are easily overlooked, especially on rough, fresh surfaces; hence it is essential that the rock be exam- ined carefully in order to place stone of consistently high quality on the market. Inclusions. Scattered, fine-grained inclusions, which vary in diameter from an inch to a foot, are exposed in some quarries. The contacts between these mineral aggre- gates and the enclosing rock are sharp. Microscopic study reveals that they have a well-developed granulitic texture ; the grains, about the size of granulated sugar, are smoothly in contact with each other. A few of the inclu- sions, on the other hand, have textures in which the grain contacts are interlocking rather than smooth. All the inclusions contain a slightly higher proportion of dark minerals than the rock that encloses them. They are best termed hypersthene granulites, as their principal mafic mineral is hypersthene. Further study is necessary to determine whether these are inclusions of foreign rock, or of an earlier solidified portion of the same igneous mass, or whether both types of inclusions are represented. Spots. Small, dark green spots of fine-grained min- erals are particularly troublesome imperfections. Most are about the size of a match head or a fingertip. Some are very widely scattered, but others are so closely spaced that it is difficult to cut a headstone that is completely free of spots. These spots are of several types. Some are granu- lar aggregates of hypersthene and magnetite, some are concentrations of green biotite and blue hornblende, others are compact groups of magnetite and biotite with some hypersthene, and still others are patches of fine- grained pyrite 2 mm. to 4 mm. in diameter. The concentrations of green biotite and blue horn- blende replace the earlier feldspars and pyroxenes, and many have a core of a soft, brown mineral tentatively identified as iddingsite. Such knots are particularly ob- jectionable on polished faces. If exposed to the air, the patches of pyrite oxidize rapidly and yield a brown stain. Coarse Clots. Some coarse-grained mineral aggre- gates, as much as six inches in diameter, consist mainly of hornblende and plagioclase feldspar in grains about 1.5 cm. in average length. Some pyrite also is present. Hydrothermal Alteration It already has been noted that several minerals in the rocks were formed after the period of magmatic crystal- lization. They appear in the interstices of the rock, or as replacements of the earlier rock-forming minerals, and in veins that cut across the rocks. In an old pit at the top of a hill a quarter mile south- east of the Stridsburg quarry, pieces of completely altered rock were found in the piles of waste material. The frag- ments consist mainly of epidote, with smaller amounts of quartz, zoisite, sericite, hydromica, kaolinite, blue horn- blende, chlorite, biotite, and sphene. Unfortunately, no exposures of this altered rock could be seen in the pit. The many veins exposed in cuts along the service road from the Stridsburg quarry to the top of the hill are so abund- ant that they could well be called a stockwork. The veins vary in composition, and include thin white seams of zeolite minerals, irregular layers of fine-grained black tourmaline and epidote in a matrix of quartz and coarser epidote, and veins of almost pure epidote. The rocks of the Stridsburg quarry have been dif- ferentially altered by hot solutions. The textural relations and association of the sulphide minerals with epidote and blue hornblende indicate that the sulphides were formed contemporaneously with other hydrothermal minerals. The proportion of secondary minerals in the rock is greater in centers of hydrothermal activity such as stock- works of vein material. The rock in th'e Stridsburg quarry, about 200 yards from the stockwork, contains more pyrite, epidote, sphene, blue hornblende, and saussuritized feld- spar than that in the Valley Granite Company and Pacific Cut Stone (piarries to the north and northwest. Commercial "Black Granite" op San Diego County In ft- M* Figure 10. Transported boulders, Emil Johnson and Sons quarry, Pala. Photo by K. H. J alma. Hydrothermal alteration is very strong in some zones of intense fracturing. A steeply dipping shear zone about fifteen inches wide is exposed in the quarry of the Valley Granite Company. The crushed and sliced rock has been almost completely altered to sericite and biotite, with hydromica, kaolinite, epidote, zoisite, and quartz. Large split boulders found on the dumps of the quar- ries near Vista contained paper-thin seams of epidote. These veinlets not only mar the appearance of the rock, but they also define surfaces along which the rock will break easily. A thin seam of epidote is exposed in the granodiorite of the Cameron-Deering quarry. The rock is bleached on both sides of the veinlet to a distance of 5 to 6 millimeters. Under the microscope it is evident that the bleached area consists of a highly altered rock in which biotite has been replaced by chlorite and epidote. The plagioclase is al- tered to saussurite, an aggregate of fine-grained zoisite, epidote, and albite, with variable amounts of calcite, sericite, chlorite, and actinolite. The quartz is unaltered. Sheet Structure, Joints and Other Types of Fractures It is difficult to observe such features as joints, frac- tures, and sheet structure in quarries that have been developed in residual material. Where solid ledges are ex- posed, however, distinct joints, fractures, and a less defi- nite sheeting are visible. In the San Marcos gabbro a consistent joint set varies in trend from N. 20° E. to N. 50° E. In many places a sub- ordinate joint set trends at right angles to the stronger northeast set. The joints dip steeply, and frequently are vertical. A very prominent joint set that strikes N. 70° W. and dips 75° S. is present in the granodiorite of the Cameron- Deering quarry. A minor set of joints crosses the main group at right angles. The size of joint blocks varies considerably, even in a single quarry, and blocks range in width from 1 to 10 feet. In general, the joint blocks are smaller near the surface than at greater depths. In part this increase in size is only apparent, because some of the joint planes, which are etched out by weathering near the surface, become tighter with increasing depth until they are barely discernible or even are invisible. The sheeting surfaces are gently inclined to hori- zontal, and are parallel or subparallel with the slope of the land surface (fig. 11 ) . Their spacing is very irregular, but in general they are closer together near the surface than at greater depths. The distance between sheets ranges from 6 inches to about 6 feet ; it averages about 4 feet. In addition to joints and sheeting, some irregular fracture surfaces are present (fig. 12). These vary con- siderably in attitude and distribution. In some places fracturing is so closely spaced that the rock is cut into irregular blocks too small to be used as monument stock. The presence of joints, sheeting, and fractures is a very important factor in quarrying and utilization of the rock. If these features are only a few inches apart, the rock is useless as dimension stone. On the other hand, if they are spaced much more than 30 feet apart, the blocks are too large to handle efficiently. The distribution of both weathering and hydrothermal alteration appears to have been controlled in large part by these features. Faulting The removal of rock from some of the quarries has exposed steeply dipping faults characterized by thin gouge zones. In many places, vertical slickensides are on the outer surface of the gouge, next to the fresh wall rock. The latest movement along the faults probably was dip slip in nature, but the amount of displacement was not determined. At one quarry the rock adjoining the footwall of a prominent fault was found to contain small, closely Figuke 11. Sheet structure at Simpson quarry, Santee. 16 Special Report 3 spaced fractures that were clearly visible on a polished surface. Examination of this material under the micro- scope showed that the rock had been strained, as evidenced by the presence of tiny fractures and bent twin lamellae in feldspar crystals. Figure 12. Partly weathered rock traversed by numerous fractures. Emil Johnson and Sons quarry, Vista. Rift and Grain Directions of easy splitting are very difficult to find in the black granite, especially in quarries developed in residual boulders. The poor development or lack of rift and grain directions is typical of black granites, and these rocks are therefore more difficult to quarry than most true granites. This difficulty is illustrated by the presence in the quarry dumps of many discarded blocks that are bounded by sharply curved surfaces rather than by flat surfaces, made when the stone was split by we'dging. Rift and grain are much better developed in the granodiorite than in the black granite. The ease with which the grano- diorite can be split makes it possible to utilize the stone for paving blocks. A study of hand specimens and thin sections revealed no evidence of preferred orientation of mineral grains or of the presence of rows of bubbles or microscopic cracks that might account for rift and grain. Much more study is needed, however, and the examination of oriented thin sections and the application of universal stage methods might be particularly valuable. In all the places where directions of easy splitting in the black granite were reported by quarrymen, these directions are parallel to joint and sheeting surfaces. It is possible, therefore, that any rift and grain present is related to incipient jointing and sheeting. 15 Weathering The residual boulders common in this region have been formed chiefly by disintegration. Microscopic study of material in all degrees of decay shows that the process of weathering begins with the oxidation and some hydra- tion of the ferrous iron in the sulphide minerals and ferro- magnesian minerals, with formation of limonite, a brown '•*■ Bowles, Oliver, op. ctt., p. 109, 1939. iron oxide. The increase in volume caused by development of this oxide has resulted in fracturing of the feldspars, and the rock has been broken into a crumbly, granular mass. Some of the iron oxide has migrated from the source minerals to fractures in the feldspars. Hydration of the feldspar probably begins after breakdown of the ferromagnesian minerals, and is notice- able only in the more advanced stages of weathering. Hal- loysite and kaolinite form along the fractures and cleav- age planes. The more calcic cores of the zoned plagioclase crystals are particularly susceptible to alteration, and many consist entirely of halloysite, whereas the more sodic margins remain relatively fresh. The alteration of the feldspars is rarely quantitatively important in these rocks, however, although the alteration that does occur probably contributes to the mechanical breakdown of the rock. The sulphide minerals are most susceptible to oxida- tion, and become completely altered within a few years or tens of years after exposure to weathering. Other min- erals, listed in order of decreasing susceptibility to oxida- tion, are hypersthene, augite, hornblende, and biotite. Biotite is locally altered to vermiculite. Magnetite is little affected by weathering. Figure 13. View into Cameron-Deering quarry, Lakeside. Distribu- tion of the many fractures in the rock is such that uniform blocks are difficult to obtain. In the black granite quarries the gradation between fresh boulders and weathered material is abrupt. The boulders are bordered by freshly spalled material, Some of these spalls are several inches thick. Their outer sur- faces are strongly stained with iron oxides, but the inte- rior parts of the spalls are still fresh. Farther from the centers of the boulders, the spalls are thinner and more completely altered. If exposure to weathering has been sufficiently prolonged, the rock becomes a granular mass in which the original concentric shells or onionskin struc- tures are only faintly visible. Several stages of weathering were observed in the field. Incipient weathering along joints and fractures in massive rock is the first stage. In the intermediate stages, residual boulders are separated by spalls and granular material, and in the final stage only a crumbly mass remains, with perhaps a few fist-sized remnants of fresh rock. Commercial "Black Granite" oe San Diego County 17 Figure 14. Perfect joint plane. Cameron- Deering quarry, Lakeside. The granodiorite at the Cameron-Deering quarry has not developed the spalls that are typical of most of the black granite exposures. The rock is stained brown as much as six inches from the joint and fracture surfaces. Here again alteration of ferromagnesian minerals, chiefly biotite, has produced brown iron oxides. The quartz and feldspar grains are fractured, and the fractures serve as avenues along which the iron oxide has migrated. The feldspars are slightly altered, but it is only along and near the joints that the rock has begun to disintegrate. The absence of spalling probably is due to the very small amount of ferromagnesian minerals and iron sulphides present, as compared with the amount in the black granite. Not enough brown iron oxide forms to break up the grains. Here the hydration of the feldspars is probably the main cause of disintegration. The degree of weathering varies considerably throughout the region, and the thickness of the weathered rock ranges from a few inches to scores or even hundreds of feet. In a few places re-entrants of intensely weathered rock 10 to 20 feet deep penetrate the fresh rock. The wea- thered material consists almost entirely of iron oxide and clay. These re-entrants are along highly sheared zones, where the relatively high porosity and permeability afford greater accessibility for the downward percolating water. The roots of plants are present throughout these zones, and undoubtedly have helped accelerate the process of alteration by further fracturing the rock, and by increasing the chemical activity of the weathering solu- tions by contributing organic acids. The depth and extent of weathering depend upon many factors, including (1) spacing of joints, sheeting, fractures, shear zones, and faults, (2) geomorphic history of the area, (3) rainfall, and (4) vegetation. Rock that is cut by many closely spaced joints, sheet- ing, fractures, shear zones, and faults is much more deeply weathered during a given length of time than is a more massive rock. The rock in the high mountains, such as the Aqua Tibia, is more deeply weathered than that in the lower rolling hills, because there is much greater rainfall and heavier vegetation in the mountains. 10 The distribution and extent of old erosion surfaces markedly affect the distribution of weathered rock. These surfaces occur at several different levels in San Diego County. Some are preserved only as small remnants, and are represented by terraces and benches along the sides of canyons and valleys. Other surfaces are still fairly exten- sive, covering as much as 30 square miles. The Fallbrook and Escondido Plains, Bear Valley, and Ramona Valley are typical of the larger erosional surfaces. Most of these surfaces are underlain by decomposed bedrock, although some overlie alluvium. The surfaces are rather smooth, with some low hills and a few rock outcrops, or monad- nocks. Some of these monadnocks contain large residual boulders which are quarried. The Pete Matson quarry near Vista is in a knob of black granite that protrudes above the Escondido Plain. In general, the best exposures of fresh black granite are scattered along the sides and in the bottoms of gullies and creek beds, where the agents of erosion have removed much of the loose, granular material. Figure If Valley Granite Company quarry, Escondido. General view of quarry workings. STAINING OF MONUMENTS IN SAN DIEGO CEMETERIES Owing to stains developed on black-granite monu- ments in several San Diego cemeteries, the use of this stone was for a time considerably curtailed. A study of these monuments was made to determine whether the staining actually is greater on the black granite than on other stones, or whether it appears to be greater because of the darker color of the rock. The staining was found to be of two types: (1) a brown stain formed bv oxidation of ferromagnesian "•Larsen, E. S., Jr., op. cit., p. 114, 1948. 18 Special Report 3 Figure 16. Vertical west face of McGee quarry, near Pala, as It appeared in July 1947. Note large boulders in upper part of thick, well-defined "creep" layer, which also contains smaller boulders that show some exfoliation. The underlying residual material, with its large, spheroidally weathered boulders, is well exposed to the right of the man. Its sharp upper contact is elsewhere concealed by slumped debris and quarry waste. Prepared from a sketch by R. H. Jahns. silicates and iron sulphide minerals in the rock, and (2) a white coating of extremely fine-grained material that does not appear to be the result of oxidation. Four cemeteries were visited. Of these, one ( Cemetery "A") is not being watered artificially, so that only rain- water touches the stones. The other three cemeteries are heavily sprinkled. Cemetery "B" receives water from the municipal water system, whereas Cemetery "C" and Cemetery " D " are watered from local wells. Analyses of the water used in these three cemeteries are recorded in table 4. These waters contain a large amount of dissolved solids. Table Ji, Analyses of ivater from cemeteries in San Diego area * Total solids Hardness (soap method) Bicarbonate (alkalinity) Silica. Iron oxide and alumina Calcium Magnesium Sodium . _• Chloride Sulphate pH value Cemetery 493ppm 521ppm 784ppm 220 188 346 266 237 308 5 9 16 1 2 1 46 51 109 23 16 12 74 92 77 77 106 185 21 18 16 7.12 7.08 7.06 Analyses by Clarkson Chemical Laboratory, San Diego, California. Description of stainina in individual cemeteries: Cemetery "A" — The stones in Cemetery "A" are relatively fresh. Very little brown iron staining is present, and there is little or no white coating. The feldspars in the stones are only slightly dulled. Cemetery "B" — The monuments in Cemetery "B" show a slight to moderate development of white coatings. Although these are more noticeable on the darker stones, there is no indication that they are thicker or more wide- spread on one kind of stone than on another. The coatings are most prominent on the upper parts of the stones and along the edges of the letters and numerals of the inscrip- tions. The white material is easily scraped off, and the rock underneath is fresh. Brown staining and dulling of the feldspars is only slightly more advanced than at Cemetery "A." Cemetery "C" — The stones in Cemetery "C" are strongly stained brown. The degree of oxidation is greater than at the other cemeteries. Much of the brown material forms a coating that can be scraped off. White coatings are present, but are not as well developed as those in Cemetery "B." There are indications that the water used in sprin- kling is a direct cause of the staining. The locations of individual sprinklers can be correlated with the distribu- tion of the stains on the faces of the stones. At one loca- tion the sprinkler is always placed in front and to the right of a monument. The lower right hand corner of this stone is strongly stained brown, but the upper left hand corner is little affected. In another part of the same ceme- tery, the lower seven or eight feet of a tall, light-gray granite obelisk is strongly stained. Farther up it is white and fresh appearing. Cemetery "D" — Monuments in Cemetery "D" show, more white staining than monuments in Cemeteries "A," "B," or "C." Some stones are so completely covered that it is difficult to distinguish the color of the rock from a distance of only a few feet. Some brown staining is present, but it is much less strongly developed than that in Cemetery "C." Study of the monument stones indicates that the water being sprayed upon them in the course of routine sprinkling is directly responsible for the objectionable staining and coating. As shown in the analyses, the water in the sprinkling systems contains a large amount of dis- solved solids. The amount of staining and coating is directly proportional to the amount of solids in the water. Commercial "Black Granite" of San Diego County 19 The weathering at Cemetery "A" probably is about normal for exposure in this area. The rate of oxidation has been increased by sprinkling, due to the longer period of time that water is in contact with the monuments and to the much greater oxidizing ability of the city water and well water as compared with rainwater. The abundant solids in the water of the sprinkling systems are deposited as a coating when the water evapo* rates. The fine-grained powder from the coatings appears to consist mainly of a carbonate, with minor iron oxides, amorphous silica, and sulphate material. Although in a given time more brown iron oxide forms from alteration of minerals in the black granites than from those in granites, syenites, and granodiorites, the lighter-colored rocks show the brown staining better. On the other hand, the white coatings are more con- spicuous on the darker rocks. The staining and coating of monuments could prob- ably be greatly decreased and the fresh appearance of all varieties of stone maintained longer, either by use of a method of watering that considerably reduces the splashing and spraying of the stones, or by installation of water-softening facilities. Figure 17. Disfiguration of black granite monument. White coating has been scraped from surface to show original dark color of polished stone. CONCLUSIONS AND RECOMMENDATIONS The important geologic features of San Diego County gabbro and granodiorite, as related to the industrial use of these rocks, have been emphasized in this study. In summary, several points seem worthy of note. 18. Disfiguration accentuated on carved surface of monument. The most deleterious mineral group, so far as weath- ering of the stone is concerned, is the iron sulphides ; only a very small amount of pyrite is sufficient to be detri- mental. The recognition of areas of most intense hydro- thermal mineralization should be helpful in distinguish- ing rock that contains harmful concentrations of sul- phides, and that hence should not be quarried. Through careful examination and appraisal of rock being quarried, flaws and imperfections can be avoided, so that only good stone need be handled for marketing. The desirability of opening large ledges of massive rock is apparent. A detailed study of the distribution of old erosion surfaces and of the physiography is important in locating such ledges, as the depth of weathered mate- rial is closely related to the geomorphology of the region. Tests to determine the physical qualities of the vari- ous rocks should be conducted. Authentic information of this type, much desired by some consumers, would thus be available. Consideration of diversification of products by pro- ducers is suggested. Several possible uses for San Diego County granites have been mentioned. A study to deter- mine the feasibility of producing and marketing a varied line of products in strongly competitive fields should be made. An important factor in the continued growth of the industry will be found in production costs of quality material. Operations should be analyzed in view of re- ducing costs, both in quarries and finishing shops. Standardization of products is desirable, so that only quality stone reaches the market. Standards might include the amount of pyrite and other detrimental minerals and the kind and number of structural defects to be allowed. Staining of monuments in San Diego cemeteries has been accelerated by sprinkling. The studies of staining have shown that white coatings have been formed on all types of stone by the evaporation of' hard water and deposition of the dissolved solids. Possible remedies sug- gested are regulation of sprinkling and water treatment. 31136 8-50 2M prnileJ in California state printinc. office