298 



REPOSITORIES OF METALLIC ORES. 



after gold had been discovered in these states, the inhabitants were 

 content with searching for gold in the beds of the brooks and rivers 

 after heavy rains. One of the proprietors of a gold stream, having 

 noticed that it never yielded gold above a certain point, where a 

 small brook entered into it, was induced to trace the brook to its 

 source, and discovered in the adjacent rocks, veins of quartz which 

 were found to contain pieces of native gold, and were subsequently 

 worked as mines. It is highly probable that in Africa, the sands in 

 certain parts of rivers become auriferous, by the depositions from 

 rivulets that flow into the main stream. 



Mr. Hennah, of Plymouth, has in his collection several pieces of 

 native gold, varying from the size of a bean to that of a hazel-nut ; 

 they were found in stream works near St. Austel : he has also a spe- 

 cimen of stream tin, eight or nine inches in length, and five or six in 

 breadth, which was evidently, once, part of a vein. In the same 

 stream work they could distinguish at different depths, the different 

 veins from which the ore had been washed out. The pebbles of Xin 

 ore, have, in some situations, been washed into the sea, and after- 

 wards covered by beds of clay or gravel. In Mount's Bay, south 

 of the town of Penzance, there was formerly a bed of stream tin 

 worked under the sea. The stream tin covers the killas or slate 

 rock of the country, and is covered by a bed of clay : a perpendic- 

 ular shaft or tunnel was sunk through the clay, and the bed of stream 

 tin was worked like a bed of coal, the clay forming the roof. See 

 Plate VII. fig. 8. The workings were continued under the sea, but 

 were at length inundated and discontinued. 



The bed with pebbles of tinstone, is seen covering the beds of 

 slate ; upon this is a thick bed of water tight clay, over which the 

 tides roll. An iron cylinder was sunk through the clay as a shaft to 

 the tin stone, which was worked like a bed of coal and drawn up 

 the cylinder. 



The following is a summary account of the rocks in which the 

 different metallic ores are generally found : 



Platina and the recently discovered metals called palladium, rho- 

 dium, osmium and iridium, have been found only in the sands of 

 rivers.* 



Gold and silver are found in primary and transition rocks, in por- 

 phyry and sienite, and in the lowest sandstone. Gold has been oc- 

 casionally discovered in coal, and very abundantly in the sands of 

 rivers, and sometimes in volcanic rocks. 



Mercury is found in slate, in limestone, and in coal strata. 



Copper, in primary and transition rocks, in porphyry, sienite, and 

 occasionally in sandstone, in coal strata, and alluvial ground. Mass- 



* Boussingault discovered platina along with oxide of iron and gold in Sienite, 

 near Santa Rosa, in the province of Antioquia.— ^Ititi. de Chim. ei de Phijs. t. 32. 

 f. 209.— B. S. 



