lid M M. Junker & DvFRENOY 



wards the N.E. at aa angle of from 45° to 50° with the 

 perpendicular, whilst the others dip to the S.W. at an angle 

 of 60° to 80°, and cover the second granite of Trou du 

 Moine fou. The latter is above the schist of Port Piriac, 

 which dips under it towards the S.W. at an angle of from 

 40* to 60°, and has a direction from S.W. to N.E. which is 

 that of almost all the rocks of Brittany. It then appears 

 evident ; 1st, that the two groups we have described belong 

 to two diflFerent formations ; 2dly, that they are essentially 

 primitive, and that the first, containing indications of tin, is 

 the most ancient.* 



Oxide of Tin in place. 



Indications of tin are met with in the 400 last metres 

 [about 1312 feet] of the granitic rock, and at about 1500 

 metres [4839 feet] io the S. S. W. of Piriac ; the rocks 

 or the veins they contain are only exposed in the numerous 

 reefs projecting into the sea, which with the exception of 

 some points or summits, are covered by every tide. The re- 

 mainder of the beach to the escarpment terminating it, and 

 against which the sea breaks in bad weather, is commonly 



♦ This stanniferous district of Brittany, is most probably a continua- 

 tion of the stanniferous rocks of Cornwall, more particularly of the 

 Land's End ; it does not appear however that the schistose group con- 

 tains tin as in Cornwall; from a memoir published in the 2d vol. of the 

 Geological Tranasctions of Cornwall, by Dr. forbes, it appears that the 

 Land's End district may also be divided into two groups, the granitic 

 and schistose, the granite like that of Piriac, is stanniferous, and the 

 schistose rocks may probably be referred to the same age as those co- 

 Tering the granite in that district; greenstone occurs in the schistose 

 groups of both countries ; the schorl (tourmaline) rock of Brittany ap- 

 pears as separated from the granite by a rock of mica and quartz, where- 

 as in Cornwall it appears to be a modification of granite. With regard 

 to the other schistose rocks, felspar and hornblende appear most abun- 

 dant in Cornwall, and quartz in Brittany. 



For a particular account of the schorl rock of Devon and Cornwall, 

 consult Mr. J. J. Conybeare's Notice on the Geology of those counties, 

 (Annals of Philosophy, vol, 5. new series, p. 188 and 189.) (Trans). 



