On the Tin of Piriac. 107 



The granite sometimes presents the appearance of stratifi- 

 cation ; the beds seem to have a direction from S.E. to N.W. 

 and to dip towards the sea, i. e. towards the S.W. at a small 

 angle with the horizon. It contains but few subordinate 

 beds ; some stripes of no great thickness are seen in it of 

 very micaceous slate, or rather of mica in mass, and very 

 large grained granite, especially at Clis and Pouliguen. 



The part of this coast comprised between the Croisic to 

 about 1500 metres [4839 feet] on the S. S.W. of Piriac, 

 is still granitic, and presents nearly the same characters as 

 the preceding, for two-thirds of the distance, i. e. veins are 

 seen in it of quartz, of quartz and tourmaline (schorl ?) of 

 quartz and felspar, of graphic granite, and very fine grained 

 granite. 



As the point that we have mentioned is approached, at 

 the separation of the two primitive groups, the grain of the 

 granite becomes larger, particularly iu the last 400 metres 

 [1312 feet] which contain the oxide of tin. The 

 slightest trace of stratification is no longer observed in it ; 

 the veins become thicker and more numerous, and scarcely 

 contain any thing but crystalline quartz. All this extent of 

 coast is covered by a bed more or less thick of quartzose 

 sand ; a species of dune or arid bank of many inches in 

 height formed of this same sand, accumulated by the winds 

 and waves, borders the sea, and serves as a dyke against it. 

 Thus it is only on some masses, uncovered by the sea at 

 low water, that we have been able to make our observa- 

 tions. 



At a short distance from the separation of the two forma- 

 tions, the sandy dune begins to rise ; farther on it exposes 

 the granitic base to view, which it before masked ; it is soon 

 reduced to a few decimetres thick in sand, covering the 

 rock that becomes more precipitous, and rises above a sandy 

 beach successively washed by the tides. 



Schistose Group. 

 The granite ceases near the hamlet named Penhareng, 

 and is replaced by a suite of remarkable schistose rocks. 



