Editing California gold rush (section)

==Development of gold-recovery techniques==
Approximately four hundred million years ago, California lay at the bottom of a large sea; underwater [[volcano]]es deposited [[lava]] and minerals (including gold) onto the sea floor. By [[tectonic plates|tectonic forces]] these minerals and rocks came to the surface of the Sierra Nevada,<ref name=HillLift>{{harvb|Hill|1999|pp=169–173}}</ref> and [[erosion|eroded]]. Water carried the exposed gold downstream and deposited it in quiet gravel beds along the sides of old rivers and streams.<ref name=HillRest>{{harvb|Hill|1999|pp=94–100}}</ref>{{sfnb|Young|1970|pp=[https://archive.org/details/westernmininginf0000youn/page/106 106–108]}} The forty-niners first focused their efforts on these deposits of gold.<ref name=HillShift>{{harvb|Hill|1999|pp=105–110}}</ref>

Because the gold in the California [[gravel]] beds was so richly concentrated, early forty-niners were able to retrieve loose gold flakes and nuggets with their hands, or simply "[[gold panning|pan]]" for gold in rivers and streams.{{sfnb|Young|1970|pp=[https://archive.org/details/westernmininginf0000youn/page/108 108–110]}}<ref name=BrandsMine>{{harvb|Brands|2002|pp=[https://archive.org/details/ageofgoldcalifor00bran/page/198 198–200]}}</ref> Panning cannot take place on a large scale, and industrious miners and groups of miners graduated to [[placer mining]], using "[[Cradle (mining)|cradle]]s" and "rockers" or "long-toms"<ref name=PlacerImage>{{cite web |url=http://www.goldrushtrail.net/indexgrt.asp?p=230 |title=goldrushtrail.net |url-status=dead |archive-url=https://web.archive.org/web/20060514223457/http://www.goldrushtrail.net/indexgrt.asp?p=230 |archive-date=May 14, 2006 }}</ref> to process larger volumes of gravel.{{sfnb|Bancroft|1888|pp=[https://archive.org/details/bancrohistofcali23huberich/page/87/mode/2up 87–88]}} Miners would also engage in "coyoteing",{{sfnb|Young|1970|pp=[https://archive.org/details/westernmininginf0000youn/page/110 110–111]}} a method that involved digging a shaft {{convert|6|to|13|m|sp=us|ft}} deep into placer deposits along a stream. Tunnels were then dug in all directions to reach the richest veins of [[:wikt:pay dirt|pay dirt]].

In the most complex placer mining, groups of prospectors would divert the water from an entire river into a [[sluice]] alongside the river and then dig for gold in the newly exposed river bottom.<ref name=RawlsDiv>{{harvb|Rawls|Orsi|1999|p=[https://archive.org/details/goldenstateminin0000unse/page/90 90]}}</ref> Modern estimates are that as much as 12 million ounces<ref>The [[Troy weight]] system is traditionally used to measure precious metals, not the more familiar [[avoirdupois|avoirdupois weight]] system. The term "ounces" used in this article to refer to gold typically refers to troy ounces. There are some historical uses where, because of the age of the use, the intention is ambiguous.</ref> (370&nbsp;[[metric ton|t]]) of gold were removed in the first five years of the Gold Rush.<ref name=GoldValue>Hayes, Garry "[http://hayesg.faculty.mjc.edu/Gold_Rush.html Mining History and Geology of the California Gold Rush] {{Webarchive |url=https://web.archive.org/web/20180908174814/http://hayesg.faculty.mjc.edu/Gold_Rush.html |date=September 8, 2018 }}", Modesto Junior College (accessed September 20, 2018).</ref>

In the next stage, by 1853, [[hydraulic mining]] was used on ancient gold-bearing gravel beds on hillsides and bluffs in the goldfields.<ref name=StarrHydra>{{harvb|Starr|2005|p=[https://archive.org/details/californiahistor00star_0/page/89 89]}}</ref> In a modern style of hydraulic mining first developed in California, and later used around the world, a high-pressure hose directed a powerful stream or jet of water at gold-bearing gravel beds.<ref>Use of volumes of water in large-scale gold-mining dates at least to the time of the [[Roman Empire]]. (''See'' [http://www.mining.com/lidar-survey-discovers-roman-gold-mines-in-spain-99350/ Roman-era gold mines in Spain.] {{Webarchive |url=https://web.archive.org/web/20141129083203/http://www.mining.com/lidar-survey-discovers-roman-gold-mines-in-spain-99350/ |date=November 29, 2014 }}) Roman engineers built extensive [[Mining in Roman Britain|aqueducts and reservoirs]] above gold-bearing areas, and released the stored water in a flood so as to remove over-burden and expose gold-bearing bedrock, a process known as [[hushing]]. The bedrock was then attacked using fire and mechanical means, and volumes of water were used again to remove debris and to process the resulting ore. Examples of this Roman mining technology may be found at [[Las Médulas]] in Spain and [[Dolaucothi Gold Mines|Dolaucothi]] in South [[Wales]]. The gold recovered using these methods was used to finance the expansion of the Roman Empire. Hushing was also used in lead and tin mining in Northern [[Great Britain|Britain]] and [[Cornwall]]. There is, however, no evidence of the earlier use of hoses, nozzles and continuous jets of water in the manner developed in California during the Gold Rush.</ref> The loosened gravel and gold would then pass over sluices, with the gold settling to the bottom where it was collected. By the mid-1880s, it is estimated that {{convert|11|e6ozt|t}} of gold (worth approximately US$15&nbsp;billion at December 2010 prices) had been recovered by hydraulic mining.<ref name=GoldValue/>

A byproduct of these extraction methods was that large amounts of gravel, [[silt]], [[heavy metals]], and other pollutants went into streams and rivers.<ref>{{cite web |url=http://www.orionmagazine.org/index.php/articles/article/176/ |title=Winged Mercury and the Golden Calf |author=Solnit, R. |date=September–October 2006 |publisher=Orion Magazine |access-date=December 3, 2007 |url-status=live |archive-url=https://web.archive.org/web/20071005122256/http://www.orionmagazine.org/index.php/articles/article/176/ |archive-date=October 5, 2007 }}</ref><ref name=RawlsHyd>{{harvb|Rawls|Orsi|1999|pp=[https://archive.org/details/goldenstateminin0000unse/page/32 32–36]}}</ref> Court rulings (1882 Gold Run and 1884 [[Woodruff v. North Bloomfield Gravel Mining Company|"Sawyer Act"]]) and 1893 federal legislation limited hydraulic mining in California. {{As of | 1999}} many areas still bear the scars of hydraulic mining, since the resulting exposed earth and downstream gravel deposits do not support plant life.<ref name=RawlsGrav>{{harvb|Rawls|Orsi|1999|pp=[https://archive.org/details/goldenstateminin0000unse/page/116 116–121]}}.</ref>

After the gold rush had concluded, gold recovery operations continued. The final stage to recover loose gold was to prospect for gold that had slowly washed down into the flat river bottoms and sandbars of California's [[California Central Valley|Central Valley]] and other gold-bearing areas of California (such as [[Scott Valley]] in Siskiyou County). By the late 1890s, [[dredging]] technology (also invented in California) had become economical,<ref name=RawlsDredge>{{harvb|Rawls|Orsi|1999|p=[https://archive.org/details/goldenstateminin0000unse/page/199 199]}}</ref> and it is estimated that more than {{convert|20|e6ozt|t}} were recovered by dredging.<ref name=GoldValue/>

Both during the gold rush and in the decades that followed, gold-seekers also engaged in "hard-rock" mining, extracting the gold directly from the rock that contained it (typically [[quartz]]), usually by digging and blasting to follow and remove veins of the gold-bearing quartz.<ref name=RawlsMine>{{harvb|Rawls|Orsi|1999|pp=[https://archive.org/details/goldenstateminin0000unse/page/36 36–39]}}</ref> Once the gold-bearing rocks were brought to the surface, the rocks were crushed and the gold separated, either using separation in water, using its density difference from quartz sand, or by washing the sand over copper plates coated with [[mercury (element)|mercury]] (with which gold forms an [[amalgam (chemistry)|amalgam]]). Loss of mercury in the amalgamation process was a [[Mercury contamination in California waterways|source of environmental contamination]].<ref name=RawlsStamp>{{harvb|Rawls|Orsi|1999|pp=[https://archive.org/details/goldenstateminin0000unse/page/39 39–43]}}</ref> Eventually, hard-rock mining became the single largest source of gold produced in the [[Gold Country]].<ref name=GoldValue/><ref>{{cite web |url=http://pubs.usgs.gov/fs/2005/3014/ |title=Mercury Contamination from Historical Gold Mining in California |access-date=February 26, 2008 |author1=Charles N. Alpers |author2=Michael P. Hunerlach |author3=Jason T. May |author4=Roger L. Hothem |work=[[U.S. Geological Survey]] |archive-date=February 22, 2008 |archive-url=https://web.archive.org/web/20080222034946/http://pubs.usgs.gov/fs/2005/3014/ |url-status=live }}</ref> The total production of gold in California from then until now is estimated at {{convert|118|e6ozt|t}}.<ref name=GoldInCal>{{cite web |last=Hausel |first=Dan |title=California – Gold, Geology & Prospecting |url=http://californiangold.blogspot.dk/ |access-date=February 19, 2013 |archive-date=October 12, 2014 |archive-url=https://web.archive.org/web/20141012202722/http://californiangold.blogspot.dk/ |url-status=live }}</ref>

===Image gallery===
<gallery mode="packed" widths="140" heights="180">
File:Gullgraver 1850 California.jpg|Forty-niner panning for gold
File:Wooden gold sluice in California between 1890 and 1915..jpg|Sluice for separation of gold from dirt using water
File:Gold seeking river operations California.jpg|Excavating a riverbed after the water has been diverted
File:Quartz Stamp Mill.jpg|Crushing [[quartz]] ore prior to washing out gold
File:California gold miners with long tom (cropped).jpg|California gold miners with long tom, {{Circa|1850–1852|lk=no}}
File:Mining on the American River near Sacramento, circa 1852.jpg|Mining on the American River near Sacramento, {{Circa|1852|lk=no}}
File:River mining, North Fork of the American River, California (cropped).jpg|River mining, North Fork of the American River, {{Circa|1850–1855|lk=no}}
File:Hydraulic mining in Dutch Flat, California, between 1857 and 1870.jpg|Excavating a gravel bed with jets, {{Circa|1863|lk=no}}
File:Panning on the Mokelumne.jpg|Panning on the [[Mokelumne River]] (1860 illustration)
File:Chinese Gold Miners b.jpg|Chinese gold miners in California (illustration)
File:Henry Raschen - California Miner with Pack Horse.jpg|[[Henry Raschen]], ''California Miner with Pack Horse'', 1887, oil on canvas
</gallery>