OLDER ROCKS OF BRITTANY. 



311 



enable us to say. I will only remark that it is difficult to explain 

 this marked mineral banding without assuming some original differ- 

 ences of arrangement in the constituents of the primary rock. 



These gneissose rocks form the irregular coast-line to the sandy 

 bay of Kerselec, but we find an interesting variation before reaching 

 a headland occupied by an old fort. Here is a band of mica-schist, 

 perhaps 40 feet thick ; it distinctly underlies the banded series 

 already described, and overlies a less banded series resembling a 

 coarser part of the upper rock, but rather greener in colour. A 

 similar rock may be seen at the top of the cliffs near a signal-station, 

 and by the fort just beyond is a darkish green gneissic rock*. The 

 mica-schist has a general resemblance to that at the Start Point 

 (S. Devon), and has obviously undergone cleavage-foliation. This, 

 however, is parallel with the bedding of the rockt. In the sandy 

 bay above mentioned we have more or less banded gneissic rock 

 resembling that which is already described. 



Bands of " amphibolite " occur at intervals along the coast. 

 These, according to the French map, are three in number, and two 

 of them are marked as extending from the embouchure of the river 

 to the sandy bay. Between these is indicated a band of serpen- 

 tine ; this, however, I did not see. The amphibolites appeared to 

 me undoubtedly intrusive rocks ; for though they had a general con- 

 cordance with the mineral banding of the gneiss, I occasionally 

 detected them distinctly cutting across it (fig. 2), and I further 



Fig. 2. — At the Embouchure of the Poulclu. 



A. Banded gneiss, the darker 

 shades representing the more 

 micaceous layers. 



B. Amphibolite, crushed and schistose, 

 but far less regularly banded than A. 



believe that the gneiss was already foliated when the intrusion took 

 place, but that the mechanical disturbance which has affected the 

 gneiss has produced distinct schistosity in the amphibolites. 



I have examined specimens of two of these amphibolites from the 

 western side of the peninsula, one rather coarse, the other compact 



* The former of these, and perhaps also the latter, is very likely a continua- 

 tion of the beds mentioned on the shore, but I had not time to prove this. 



t Owing to the fissile character of the rock it is practically impossible to 

 obtain a sufficiently thin section for accurate microscopic examination. Parts 

 of it are almost opaque from the abundance of a black mineral, which, as in the 

 specimens from the Start, is probably partly graphite, partly iron oxide ; there 

 is a white mica, and some brown, and. a fair amount of a dichroic greenish 

 mineral, rather like the chloritoid of the He de Groix. 



