OLDER ROCKS OP BRITTANY. 



317 



the gneiss, it is bordered by a quartz-felspar vein, sometimes banded 

 with tourmaline. The " vein " seems to " muddle up " with the 

 granite, but is sharply divided from the gneiss, into which it sends an 

 offshoot. This granite exhibits no signs of mechanical disturbance. 



(4) The amphibolite is a dark hornblendic rock, sometimes con- 

 taining black mica. In some places it seems to be interstratified 

 with the gneissic rock, but as the thickness is variable, and it occa- 

 sionally cuts obliquely across the bands of the other, I consider it 

 intrusive, though it not seldom follows planes of an earlier foliation. 

 A more or less foliated structure, which* is parallel with that domi- 

 nant in the gneiss, is visible in the different masses ; there is no 

 indication of a contact-metamorphism in the latter, and I fully 

 believe that the gneiss was a foliated rock when the " amphibolite " 

 was intruded, but that both were afterwards subjected to considerable 

 pressure. A specimen of the amphibolite, which macroscopically 

 closely resembles the darker more homogeneous varieties of the 

 hornblende-schist at the Lizard, has for its dominant mineral a dark- 

 green hornblende, exhibiting strong dichroism. The cleavages 

 parallel to cop are well developed, and crystal faces cop and 

 oop x oo are not rare. The grains commonly measure "02", or a 

 little less, in the direction of the vertical axis, and rather less than 

 •01" in the direction of the clino-diagonal. A colourless mineral, 

 sometimes slightly " powdered " with ferrite, which has consolidated 

 after the hornblende, comes next. It is difficult to be sure whether 

 this is quartz or felspar, but the latter mineral is present. A little 

 biotite, a few grains of iron oxide, a dirty-looking mineral which I 

 think probably an impure epidote, and some small crystalline grains 

 which I take to be sphene also occur ; thus the resemblance of this 

 rock to some of the Lizard hornblende-schists is very striking. 



Gneiss and granite, as above described, are traversed by the rail- 

 way some little distance to the south of Roscoff, and are then 

 succeeded by slaty rock. Morlaix affords excellent opportunities 

 for studying the effects of pressure and of igneous intrusion on 

 ordinary sedimentary rock*. The deep and craggy valley in which 

 this quaint old town nestles by the riverside, the quarries, the road- 

 side cuttings, and even the blocks in the rough-built walls which 

 prop up the terraced gardens, afford endless studies of the results of 

 pressure. The dominant rock formerly consisted of a dark clay 

 closely interbanded with grey silt or fine earthy sand. As a rule the 

 bands of the latter do not now exceed 1", and are often thinner. The 

 former are often over 1", and sometimes the argillaceous rock is free 

 from the sandy bands. The planes of cleavage and of stratification 

 are very commonly coincident, but the bands indicative of the latter 

 constantly zigzag and wriggle across the former. The sandy bands 

 only exhibit a faint cleavage ; hundreds of examples illustrate how 

 they thicken in one part of a crumple and attenuate in another 

 under the alternating action of thrusts and strains. Cleavage is 

 highly developed in the dark bands ; its surface-planes are often 



* Dr. Le Hir (Bull. Soc. Geol. Fr. 2 e ser. t. xxviii. p. 87) considers the rock 

 about Morlaix to be of Devonian age. 



Q.J.G.S. No. 171. K 



