Comstock Lode

The Comstock Lode was a massive body of silver ore discovered under what is now Virginia City, Nevada in 1859. Between 1859 and 1878 it yielded $400 million in silver and gold. It is notable not just for the immense fortunes it generated and the large role those fortunes had in the growth of San Francisco, but also for the advances in mining technology that it spurred.

In the early 1850s, '49ers on their way to California as part of the Gold Rush discovered small placer gold deposits in the vicinity of Dayton, Nevada. These deposits were followed up Six-Mile Canyon, where veins of gold-flecked decomposed quartz underlaying a heavy blue-black sand were discovered. This sand was viewed as a nuisance until an assay determined that it was, in fact, a rich silver ore far more valuable then the gold ore beneath it. The deposits of this ore came to be collectively referred to as the 'Comstock Lode' after Henry Comstock who was the most conspicuous claim holder in the area.

News of this discovery spread quickly and soon hundreds and then thousands of people flocked to the area. The ore was first extracted through surface diggings, but these were quickly exhausted and miners had to tunnel underground to reach ore bodies.

Unlike most silver ore deposits which occur in long thin veins, those of the Comstock Lode occurred in discrete masses often hundreds of feet thick. The ore was so soft it could be removed by shovel. Although this allowed the ore to be easily excavated, the weakness of the surrounding material resulted in frequent and deadly cave-ins.

The cave-in problem was solved by the method of square-set timbering invented by Philip Deidesheimer. Previously timber sets consisting of vertical members on either side of the diggings capped by a third member were used to support the excavation. However the Comstock ore bodies were too large for this method. Instead, as ore was removed it was replaced by timbers set as a cube six feet on a side. Thus the ore body would be progressively replaced with a timber lattice. Often these voids would be re-filled with waste rock from other diggings after ore removal was complete.

As the depth of the diggings increased, the hemp ropes used to haul ore to the surface became impractical, as their self-weight became a significant fraction of their breaking load. The solution to this problem came from A. S. Hallidie in 1864 when he developed a flat woven wire rope. This wire rope went on to be used in San Francisco's famous cable cars.

Intrusion of scalding-hot water into the mines was a large problem, and the expense of water removal increased as depths increased. In 1871 the Sutro Tunnel was driven up from the valley near Nevada through nearly four miles of solid rock to meet the Comstock mines approximately 1,650 feet beneath the surface. The purpose of the tunnel was to provide drainage and ventilation for the mines as well as gravity-assisted ore removal. However by the time it reached the Comstock Lode, all of the ore above 1,650 feet had already been removed and the lower workings were 1,500 feet deeper still. The Sutro Tunnel was an engineering marvel, but a financial failure.

Peak production from the Comstock occurred in 1876, with the mines producing nearly 1,000 tons of ore a day. Production decreased rapidly thereafter, and by 1878 the Comstock was considered to be played out. The last of the water pumps was shut off in 1886, and the mines quickly flooded.

Nevada is commonly called the 'Silver State' on account of the silver produced from the Comstock Lode. However, since 1878 Nevada has been a relatively minor silver producer, with most subsequent bonanzas consisting of more gold than silver.