The economy of Napa County has traditionally been dominated by agriculture, along with a few manufacturing industries. The upper county, however, is best known for its mineral wealth. Looking at the heyday of Napa's mining era, one sees a picture dominated by a few minerals. In the long run, the two most valuable have been mercury, or quicksilver, and mineral water. But, as is usually the case, the spotlight of fame focuses most brightly on a relatively small part of the picture - silver.
Robert Louis Stevenson, in his Silverado Squatters, made famous the ghost mining town of Silverado, once located between Calistoga and Middletown, on the shoulder of Mt. St. Helena.
With a newly installed two-story stamp mill, Alex was soon processing silver ore at a rate that hinted at fabulous wealth hidden within the mountain. At its peak, during one four month period of 1874, $93,000 in silver and gold came out of this one hole in the ground. A populace barely a generation removed from the great California Gold Rush of 1849 needed no further urging to begin riddling Mt. St. Helena with holes.
Where only scrub brush and steep rocky slopes had been, an entire town sprang up. Every business necessary to separate hopeful miners from their money was represented - particularly saloons. There were several hotels and a boarding house. Rumor has it that as many as 1500 people lived on the mountain at that time. Virtually all the trees were cut down and used for firewood or for lumber and mine supports.
The fever increased when word got out that the Monitor Ledge had struck a fault and vanished. All indications were that the bulk of the deposit had yet to be mined. This meant that somewhere, maybe near the mine, maybe miles away, the rest of the Ledge must be hidden, just waiting for another lucky miner to stumble across it and become a very rich man.
The renewed frenzy of digging only exhausted the hapless miners and finished the job of covering the mountain with holes. The rest of the Ledge was never found. Within a few years the town of Silverado had disappeared, as the miners either moved the buildings to other mines or tore them down for the wood. When Stevenson and his family arrived in the summer of 1880, only the ruin of the boarding house was left.
A brief walk up the mountain from the parking lot at the Robert Louis Stevenson Memorial State Park will take you to the mine opening and the former site of the boarding house. A monument marks the spot. Now it's only a small stop on the way up to the road to the top of Mt. St. Helena. It's difficult today to imagine the hysteria that must have existed there. So much activity on so little flat ground.
In 1874, as silver production at Silverado peaked at the aforementioned $93,000, quicksilver production was just taking off, and for the year exceeded $1,000,000. The most common ore of mercury around here is a soft orange rock called cinnabar. Miners looking for gold and silver soon realized cinnabar was much more plentiful and still well worth mining.
Among the noteworthy cinnabar mines were the Oat Hill Mine, above the Palisades, and the Knoxville Mine, north of what is now Lake Berryessa. In the 1880's, the Knoxville Mine was the third largest cinnabar producer in the state. (Homestake Mines now has a large project underway at Knoxville reprocessing the mine tailings for gold.) Of 14 quicksilver mines listed in the 1927 San Francisco Field Division Report of the State Mineralogist, only 4 had any significant work still going on, and even these were winding down. The heyday of the Napa Quicksilver Mines was the 1800's.
Magnesite is an ore of magnesium, which most people nowadays know mainly as the light, strong metal that mag rims are made of, and that early photographers used as flash powder. Napa County produced 40,000 tons of magnesite in 1917, when magnesium was required for the war effort. Almost a million dollars worth came out before operations closed down in 1924. Among Napa's magnesite mines were the Priest Mine, 18 miles east of St. Helena, and the White Rock Mine, also known as the Pope Valley Mine. Before its vein of ore ran out, the White Rock Mine was one of the most important sources of magnesium ore in California.