Contra Costa, a W. county of California, bounded N. by the strait of Carquinez, Suisun bay, and the San Joaquin river, E. by the San Joaquin, W. by the bay of San Francisco, and N. W. by San Pablo bay; area, 756 sq. m.; pop. in 1870, 8,461, of whom 160 were Chinese. There are mountains in the southern part, the most considerable of which is Mt. Diablo, but the surface in other portions is generally level. The soil is fertile and productive. There are numerous salt and sulphur springs. Limestone is abundant; gypsum has been discovered; and there are quarries of red freestone and building stone of excellent quality. Valuable mines of coal have recently been opened in Mt. Diablo, and are now extensively worked. Copper and quicksilver are found. There are railroads from the coal mines to San Pablo bay. The chief productions in 1870 were 925,054 bushels of wheat, 67,025 of oats, 358,-350 of barley, 18,658 tons of hay, 64,890 lbs. of cheese, 153,936 of butter, 58,800 of wool, and 10,330 gallons of wine. There were 7,033 horses, 5,366 milch cows, 9,854 other cattle, 2,556 sheep, and 7,679 swine; 4 grist mills, 5 manufactories of saddles and harness, and 1 of agricultural implements.

Capital, Martinez.