Bibliography: Bulletin 38.

SLATE.

Slate was first produced in California in 1889. Up to and including 1910 such production was continuous, there being none between that year and 1915, when there was an output of 1,000 squares reported, valued at $5,000. Many large deposits are known in the state, especially in El Dorado, Calaveras and Mariposa counties, but the demand has been light owing principally to competition of cheaper roofing materials.

The property of the Eureka Slate Company in El Dorado has recently been taken over by the Sierra Slate Corporation of New York, and, it is stated, will be operated on a large scale. This will be the only quarry producing roofing slate, commercially, west of Pennsylvania. This Eureka roofing slate has been passed upon as one of three brands acceptable on Federal work, the other two being from Maine and Pennsylvania, respectively.

The new company expects to prepare for market from 1,000 to 3,000 squares per month. A "square" of roofing slate is a sufficient number of pieces of any size to cover 100 square feet of roof, with allowance generally for a three-inch lap. The size of the pieces of slate making up a square ranges from 7 x 9 inches to 16 x 24 inches, and the number of pieces in a "square" ranges from 85 to 686; and it is worth $3.50 to $10 per square, f. o. b. quarry, depending on quality. The Ferry Building, San Francisco, is roofed with Eureka slate.

A complete record of amount and value of slate produced in California follows:

MISCELLANEOUS STONE.

Bibliography: State Mineralogist Report XII. Bulletin 38. Miscellaneous stone is the name used throughout this report as the title for that branch of the mineral industry covering crushed rock of all kinds, granite, paving blocks, sand and gravel, and pebbles for grinding mills. The foregoing are very closely related from the standpoint of the producer. The quarry which produces granite blocks this year may have a commercial output of crushed rock next, or its product may regularly consist of both classes of material. Thus it has been found to be most satisfactory to group these items as has been done in recent reports of this Bureau. In so far as it has been possible to do so, granite and crushed rock production has been subdivided into the various uses to which the product was put. It will be noted, however, that in both instances a very large percentage of the output has been tabulated under the heading "Unclassified." This is necessary because of the fact that many of the producers have no way of telling to what specific use their rock was put after they have quarried and sold the same.

In addition to amounts produced by commercial firms, both corporations and individuals, there is hardly a county in the state but uses more or less gravel and broken rock on its roads. Of much of this, particularly in the country districts, there is no definite record. kept. Estimates have been made for some of this output, based on the mileage of roads repaired.

For the year 1915, stone shows an increase over the preceding year, but not sufficient to restore it to the level of the 1913 output. Apparently construction work is recovering from the slump in 1914, but has not fully regained its stride. The total value for 1915 was $5,011,108 as compared with $4,860,358 for 1914, and $6,168,020 for 1913.

As has been the case for several years past, Los Angeles County led all others by a wide margin, with an output valued at $1,022,134; followed by Alameda, second, with $457,381; Contra Costa, third, $397,330; and Humboldt fourth, $335,292. Sacramento, Riverside, Fresno and Sonoma were also important producers, in the order named.

The bulletin of the Association of American Portland Cement Manufacturers, November, 1915, relative to building permits during the first 11 months of 1915, indicated the following: Seven cities in California, Nevada, Utah, and Arizona showed a decrease of 34.8% in valuation from same period of 1914, November, 1915, however, showing an increase. A total of 85 cities in the entire United States

showed a net decrease up to August, followed by increases, so that there was a net increase of 2.1% for 11 months over 1914.

In California, the general construction situation in 1915 appeared about normal so far as small jobs were concerned; but there were no large pieces of work done except highway contracts.

Granite.

The output of granite, particularly for building and ornamental purposes, was very materially curtailed by a strike of the granite cutters which started in June and lasted practically throughout the balance of the year.

Granite Production, by Countles, for 1915.

The paving block industry has decreased materially of recent years, because of the increased construction of smoother pavements demanded by motor vehicle traffic. The blocks made in Solano County are of basalt; those from Sonoma of basalt, andesite and some trachyte; while those from all the other counties shown in the tabulation, are of granite.

Paving Block Production, by Counties, for 1915.

Grinding Mill Pebbles.

For the first time we are able to record a production of pebbles for tube and other grinding mills. Owing to the decreased imports and higher prices of Belgium and other European flint pebbles, there has been a serious inquiry for domestic sources of supply.

One shipment was made of pebbles selected from gold dredger tailings in Sacramento County, for use in a gold mill in Amador County employing Hardinge mills. It is stated that the consumption was about 3 to 1 as compared to Danish pebbles; and that the long wagon haul, coupled with the heavy consumption, made the cost not economic. Had the pebbles been selected by men with some knowledge of rocks,

Grinding mill pebbles, on beach at Bird Rock, near San Diego, California.

instead of by inexperienced laborers as in this case, a better grade would have been obtained, which no doubt would have proven of better service.

The important development in this item, however, has been in San Diego County. At several points along the ocean shore from Encinitas south to near San Diego, there are beaches of washed pebbles varying from 1 inch to 6 inches in diameter. At one of these localities visited by the writer in May, 1916, there is a conglomerate stratum forming a part of the sea-cliff. This conglomerate is made up of well-rounded water-worn pebbles of various granitic and porphyritic rocks with some felsite and flint. The wave action has broken down portions of the

cliffs for considerable distances and formed beaches of the pebbles, which are well washed and cleaned of the softer materials. The rocks sorted out for shipment are mainly basalt and diabase, with an occasional felsite and flint pebble. There is a tough, black basalt which is stated to be giving satisfactory results. Shipments are being made to metallurgical plants in California, Nevada and Montana.