Some of us keep watching the skies, as the old movie told us to do. Geologists watch the ground instead. Really looking at what's around us is the heart of good science. It's also the best way to start a rock collection, or to strike gold.
The late Stephen Jay Gould told a story about his visit to Olduvai Gorge, where the Leakey Institute digs up ancient human fossils. Institute staffers were attuned to the mammals whose fossil bones occur there; they could spot a mouse tooth from several meters away. Gould was a snail specialist, and he didn't find a single mammal fossil during his week there. Instead he turned up the first fossil snail ever recorded at Olduvai! Truly, you see what you look for.
Horn Silver and the Nevada Rush
I was thinking these things not long ago as I was breaking ground in the back yard. In one place was a whole surface of rough, weathered concrete, a thin crust just under the soil. It reminded me of the legendary horn-silver beds of Nevada: treasure that looked like concrete.
The Nevada silver rush, which began in 1858, may be the truest example of a gold rush. In the California gold rush, like those before and after, the Forty-Niners swarmed into the land and panned the easy nuggets from the stream placers. Then the geologic pros moved in to finish the job. The mining corporations and hydraulic syndicates thrived on the deep veins and low-pay ores that the panners couldn't touch. Mining camps like Grass Valley had a chance to grow into mining towns, then into stable communities with farms and merchants and libraries.
Not in Nevada. Silver there formed strictly on the surface. Over millions of years of desert conditions, silver sulfide minerals weathered out of their volcanic host rocks and slowly turned, under the influence of rainwater, to silver chloride. The climate of Nevada concentrated this silver ore in supergene enrichments. These heavy gray crusts were often polished by dust and wind to the dull luster of a cow horn—horn silver. You could shovel it right off the ground, and you didn't need a Ph.D. to find it. And once it was gone, there was little or nothing left beneath for the hard-rock miner.
A big silver bed could be tens of meters wide and more than a kilometer long, and that crust on the ground was worth up to $27,000 a ton in 1860s dollars. The territory of Nevada, along with the states around it, was picked clean in a few decades. The miners would have done it faster, but there were dozens of remote ranges to prospect on foot, and the climate was so damnably harsh. Only the Comstock Lode supported silver mining by large combines, and it was depleted by the 1890s. It supported a federal mint in Nevada's capital, Carson City, which made silver coins with the "CC" mint mark.
Mementoes of the Silver State
In any one place, the "surface bonanzas" lasted only a few seasons, long enough to put up saloons and not much else. The rough, violent life of so many Western movies reached its purest state in the Nevada silver camps, and the economy and politics of the state have been deeply marked ever since. They don't shovel silver off the ground any more but sweep it instead, off the tables of Las Vegas and Reno.
Nevada horn silver seems to be gone forever. I've looked around the Web for specimens many times, but nothing panned out. You can find silver chloride on the Web under its mineral name of chlorargyrite or cerargyrite, but the specimens aren't horn silver, even though that's what "cerargyrite" means in scientific Latin. They're little crystals from underground mines, and the sellers seem apologetic about how unexciting they look.
I would pay to have a piece of the real stuff to remind me of that unique bit of American history. Imagine picking up chunks of silver, a fortune's worth, right off the ground, looking just like that old concrete in my yard. Maybe I should go back and take a closer look at it.