on ihe Tin of Piriac. Ill 



covered by A thick bed of sand, which, in many places, 

 is interrupted by masses of granite or salient veins of quartz, 

 harder and tougher than the rest, and which have resisted 

 the effects of the waves and decomposition. 



About the equinoxes, and especially that of Autumn, it 

 sometimes happens that the sea removes this bed of sand and 

 graveli, and when retiring shews the structure and mode of 

 occurrence of the veins that traverse the ground in every 

 direction., But shortly (generally at the next tide) the 

 same sands are brought back again ; and the inhabitants of 

 Piriac are the only persons who can profit of these valuable 

 moments, in order to observe the veins, and collect without 

 trouble, the fragments of ore broken off from their places, 

 and washed by the sea. 



[In order to ascertain the thickness of the sand and ta 

 study the rock beneath, the authors opened a trench 200 

 metres [about 656 feet] long parallel to the west, and ex- 

 tending to the separation of the granitic and schistose rocks]. 



The sand covering the rock being about three or four de- 

 cimetres [about 12 to 16 in.] thick, was soon traversed. We 

 constantly found in the inequalities of the granite surface, 

 that it covered tin sand, and pebbles having a mean diame- 

 ter of two to three millimetres [about |^ in.], many of whicli 

 still shewed crystalline forms. 



The granite occurring beneath is generally very soft, and 

 the felspar in the state of Kaolin. From this circumstance 

 it has been incorrectly stated that the oxide of tin was 

 found in clay at Piriac. This granite and the rocky masses 

 advancing into the sea, are traversed by numerous veins of 

 greyish-white and fetid quartz, often containing crystals of 

 felspar and mica. These veins are of very variable dimen- 

 sions, being from one and two centimetres [about j in.] t» 

 many metres in thickness. Their direction and inclination 

 also varies, they cross each other in every direction ; often 

 swell out considerably, forming with each other a true 

 stockwerck, analogous to those of Geyer in Saxony, and 

 affording a disposition resembling that of the carbonate of 



