318 



PEOF. T. Gt. BONNET ON THE 



satiny, and frequently rumpled or wavy. A minute filmy mica 

 (sericite ?) has been developed, with rutile &c. ; but every change is 

 micromineralogical, not greater than may be seen in the Palaeozoic 

 rocks of Wales or of S. W. England, in the J urassic or Carboniferous 

 slates of the Alps, or in the Cambrian of the Ardennes. The rock, 

 at most, can be called a phyllite ; it is perfectly distinct from one of 

 the true crystalline schists already described. Still it is both inter- 

 esting and instructive to observe how close are the superficial struc- 

 tural resemblances of these banded slaty rocks and some of the 

 banded crystalline schists. Rocks might be found at Morlaix which 

 in. a photograph (but only in a photograph) would be undistinguish- 

 able from some of the banded and, presumably, less ancient schists 

 of the Alps or other similar regions. In one quarry on the Huel- 

 goat road (near the town) I obtained an excellent example of a 

 second cleavage, formed by a crumpling of the ordinary cleavage- 

 layers, which here very probably agree with the stratification— a 

 true "AusweicJiungsdivage." 



The effects of contact metamorphism are also beautifully exhibited 

 in many places. Sometimes, however, it is surprisingly slight. A 

 dyke of felsite north of the town* barely indurates and does not 

 bleach the dark slaty rock adherent to it ; in another place, east of 

 the town, we find a vein of moderately coarse granite converting a 

 dull grey slate into a greenish-coloured " porcellanite." About 

 Chapel du Mur, chiastolite and a minute black mica are developed ; 

 and in one place I found, in a black " phyllite," crystals of chiastolite 

 full -2" wide and an inch or more longf. The Huelgoat road also 

 affords some excellent illustrations of contact-metamorphism, which 

 I have examined with the microscope. They lead, however, into 

 more than one interesting, but rather wide-reaching question, so 

 that I think it better on the present occasion to content myself with 

 this brief notice, and to recur to them in a future communication. 

 Suffice it to say that the mineral changes in the most highly altered 

 among them resemble those which have been described in the 

 Skiddaw slates near the granite of Sinen Gill, and that these results 

 of contact-metamorphism differ greatly from the gneisses and schists 

 commonly considered of Archaean age, and such as are described in 

 this paper, though possibly they may furnish us with some valuable 

 suggestions as to the genesis of the latter. 



Conclusion. 



My work in Brittany, of which I have now described the more , 

 complete portions, leads me to the following conclusions : — 



(1) That the great central folded trough of Lower Palaeozoic;) 



* The dyke is about 4 feet thick. In the field, I took it for a microgranulite, 

 but the part in contact with the slate appears, under the microscope, to have 

 been, if not glassy, not more than cryptocrystalline. The junction and some 

 included fragments of the slate prove that to have been cleaved and to have 

 become a "phyllite" prior to the intrusion. 



t I failed to find good sections, and chiefly examined loose blocks, so cannot 

 say how near these were to intrusive rock. These rocks had a general resem- 

 blance to the altered rocks of Skiddaw. 



