INTRODUCTION. xvii 



§ 13. Cement Mills. — Cement mills have been brought into use since 

 1862. At the bottom of some of the ancient channels, the richest pay is 

 found in a stratum of clay and gravel, so cemented together that it will 

 not dissolve in the sluice ; and the only mode of separating the gold from the 

 dirt, heretofore, has been crushing in a stamp mill. Many of these mills 

 are now in use, and their number is increasing rapidly. The gold is 

 usually coarse in the cement. The material is not so hard as quartz, nor 

 is it crushed so fine ; so the crushing does not usually cost over 50 cents 

 per ton. A pan has lately been invented for reducing cement. It is an 

 iron pan, six feet in diameter, and eighteen inches deep, in which four 

 iron rakes, radiating from the center, each with three strong teeth, revolve 

 rapidly. A large stream of water pours in, and the pulverized stuff is 

 carried off through small holes in the bottom. The large stones are dis- 

 charged through a gate. There are two sluices, one to carry off the stones, 

 and the other to carry off the dissolved clay. 



§ 14. Copper Mining. — Copper mining in California began in 1861, and 

 was very profitable for a time, but the price of the metal has fallen greatly 

 within the last year or two, and copper mining in this State has declined. 

 The amount of ore exported in 1862 was 3,660 tons ; in 1863, -5,553 tons ; 

 in 1864, 10,234 tons; in 1865, lt,78t tons; in 1866, 19,813 tons; and in 

 the first half of 1867, 3,542 tons. Besides this ore, about 200 tons of 

 metallic copper have been shipped. The ore exported contained on an 

 average about 14 per cent, of metal. 



§ 15. Coal Mining. — Coal mining, at Monte Diablo, in California, com- 

 menced in 1861, and advanced very rapidly. In 1862, 23,000 tons were 

 produced, and in 1866, 84,000 tons. It was at first supposed that the coal 

 was of very poor quality, and that the supply would soon give out ; but 

 these suppositions have proved to be erroneous. 



§ 16. Borax. — The production of borax was commenced in California 

 in 1865, in which year 1,707 cases, worth $22 per case, were exported; 

 in the next year, 3,171 cases were exported; and in the first half of 1867, 

 3,671 cases. 



The borax is obtained from Borax Lake, which covers an area of 200 

 acres, one mile from the eastern end of Clear Lake, in Lake County. The 

 water of the lake is strong with borate of soda, and the mud at the bottom 

 of the lake is full of crystals of borax, nearly pure. 



§ 17. Other Minerals.— In 1866, 5,000 barrels of California petroleum 

 were produced. Several tons of sulphur were refined near Clear Lake; 

 and several tons of plumbago were refined at Sonora. An opal mine was 



