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Mining became such an important part of life in the Devon that as early as the 12th century, tin miners developed their own set of laws (stannary laws) and, ultimately, their own parliaments (Stannary Parliaments). These laws applied to anyone involved in the industry.
Stannaries were established in Tavistock, Ashburton and Chagford by King Edward I in 1305. Plympton followed soon after. The Devon stannary parliament met in an open air forum at Crockern Tor from 1494.
Anyone who broke a stannary law could find himself imprisoned in the gaol at Lydford. The stannary courts were abolished in 1836.
The majority of the tin mines on Dartmoor are located in granite country rock and most of the lodes trend ENE-WSW and are of limited length, though there are exceptions. In the underground workings, the tin ore, cassiterite, was usually found in association with large amounts of tourmaline, and in central Dartmoor with much specular haematite . In the southern part of the moor, the cassiterite was usually found in relatively large grains, but the lodes were of very variable quality. These factors, combined with the fact that none of the underground workings was found to be profitable at depth, are typical of the deepest zone of tin mineralisation. The once very extensive alluvial deposits of tin ore, that were the first deposits to be mined, also point to the vast quantity of ore that once existed in lodes that have been eroded from above the granite since it was emplaced in the Carboniferous era.
The earliest means of recovery, known as streaming or streamworking, involved the collection of alluvial deposits from river and stream beds where they had accumulated after being eroded from the ore-bearing lodes. The geological processes that resulted in the deposition of the cassiterite in the stream beds often resulted in very pure tin gravel which was mixed with gravels of other, unwanted, minerals such as quartz, mica and feldspar, collectively known an "gangue". It was relatively easy to separate these minerals on the basis of their very different specific gravities cassiterite about 7 and gangue 3 or less. The separation was performed by passing a stream of water over the gravels: the gangue would be washed away faster than the wanted tin gravel.
Once a tin-bearing valley had been identified, the stream-workers would arrange a stream of water, probably carried by a leat from higher up the valley, and starting at the lowest end of the deposit they would dig a trench (known as the "tye") as deep as possible to allow the finer gangue to be washed away. From there they would start working up the valley, using the stream of water to wash over the debris they had loosened from the bed with picks. This method of working leaves characteristic evidence in the valleys - a series of ridges of the larger gangue material, sometimes roughly perpendicular to, sometimes coaxial with the line of the valley, sometimes apparently haphazard, all bounded by a scarp which marked the edge of the worked ground and whose height relates to the depth of the deposits.
Eluvial deposits those that had weathered from the lode in the usual way, but had not then been transported by flowing water were also worked; these tended to be poorer deposits due to the lack of sorting that a stream provides, and they usually did not have such a ready supply of water available to work them. They are found on gently-sloping hillsides. Where possible the water needed to work these deposits was carried by a leat from the nearest available river, or if the site was above such supplies, reservoirs were constructed to collect rainwater and runoff from hillsides.
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