on the Tin of Piriic. IQ9 



bustible. This rock forms rather large veins much con- 

 torted and interlaced by veins of yellow quartz, thaii beds 

 properly so called. 



Very thin veins of yellow oxide of iron have t^epn found 

 , in it, parallel to the lamiuai. 



The sea has hollowed a very deep cavern out of this rock, 

 called Madame. 



An extremely schistose rock, composed of greenish-white 

 mica, grains of quartz and felspar, and sometimes crystal- 

 lized garnet, afterwards occupies all that part of the coast 

 comprized between the cavern of Madame and Castelli point, 

 where the shore, after having attained its greatest height, 

 10 or 12 metres [about 33 to 39 feet 6 in.], begins again 

 to get progressively lower. It is followed by the rock 

 No. 4, then by the same black quartzose rock that we have 

 just described. 



Lastly, beyond Castelli point, near a cavern named Trou 

 du Moine fou, small grained granite is found, resembling 

 that which precedes the schistose group, but of little extent; 

 it ceases in the midst of the port of Piriac, near which it 

 participates in the schistose structure of the remainder of the 

 formation. 



This granite is covered by a greenish-grey schist, at first 

 micaceous (composed of quartz and mica), but which soon 

 changes its nature, containing hornblende and passing into 

 greenstone. 



The first of these schists contains between its laminae some 

 spots of sulphuret of iron, and constitutes the surface of the 

 coast from the port of Piriac to the environs of Port au 

 Loup. The second contains garnets as an accidental mix- 

 ture, often in considerable quantity, and oxidulated iron, 

 and extends from Port au Loup to the passage of Trehiguier 

 on the Vilaine. 



AU these beds have a constant direction from S.E.toN.W. 

 nearly, presenting a remarkable disposition with regard to 

 their dip. Those which on the north follow the first stan- 

 niferous granite," to the middle of the greenish rock No. 6, 

 situated between the points of Bichet and Castelli, dip to- 



