OLDEE KOCKS OF BRITTANT. 



307 



cite?), arranged in slightly wavy bands, which, give the brightest 

 colours when placed at about 45° with the vibration-planes of the 

 crossed nicols. There are some specks of iron oxide, and one grain 

 may be brown tourmaline. Of the nature of the black rock it is 

 difficult to be sure. There is, as is common with hard, compact, 

 flinty rocks, a want of definiteness in its structure. It might 

 be a glassy igneous rock, modified by decomposition and other 

 mineral changes, but then we should have expected that it too 

 would have shown a schistose structure. The absence of this 

 suggests a difference in origin. On careful examination I fancy I 

 detect faint indications of fragments, so that I believe this to have 

 been (like so many of these halleflintas) a fine-grained tuff. 



On One point I feel certain, namely, that these two rocks have no 

 relation to the gneissic series *. The alteration in them is not more 

 than we often find in the lower Palaeozoic rocks of Britain. They 

 may be nearly parallelled by specimens from Charnwood or from 

 Carnarvonshire. They may possibly be rather older than our 

 Cambrian, but they might be later than this formation. 



Beyond this pit occurs a felspathic gneiss with cleavage-foliation 

 like that already described. 



To the south of Quimperle the coarse, strong gneiss continues for 

 some distance, and is followed by " granite mica-schisteux ; 99 then 

 comes a zone, some couple of miles wide, occupied by " mica-schistes" 

 regarded by Dr. Barrois as generally equivalent to the schistes a 

 chlorito'ide of the He de Groix. Little rock is seen by the roadside 

 till the coast is approached. I only noticed a very fissile mica-schist 

 (about 9| kilometres from Quimperle), which much resembles some 

 of the Alpine mica-schists, in which crushing has produced a cleavage- 

 foliation, obliterating all earlier structures. The strike of the 

 cleavage-foliation is roughly east to west, and the dip very high to 

 the north. But on reaching the coast a very interesting series of 

 sections is obtained at low tide, just within the embouchure of the 

 Pouldu, and along the sea-coast to the west. 



The low cliffs in the eastern part of this section afford us repeated 

 sections of a very definitely banded gneissoid rock, with some amphi- 

 bolites and a vein of felspathic granite. The first-named rock con- 

 sists of bands of a rather fine-grained gneiss, composed chiefly of 

 quartz and felspar, with a small quantity of a silvery mica of a 

 yellowish-grey colour, alternating with a rather darker and coarser- 

 grained rock ; the one somewhat resembling a vein granite, the 

 other a felspathic gneiss. The bands of the former rock vary gene- 

 rally from |" to 2" thick; those of the latter are sometimes only 

 about |" thick, but are usually more, and generally dominate over 

 the other. Both rocks, but especially the former, have a " slabby " 

 fracture parallel with the mineral banding ; in the latter, however, 



* I speak, of course, of this particular halleflinta. The halleflinta or petro- 

 silex of the " Promenade '" at Quiniper appeared to me a result of crushing of the 

 granitoid rock which forms the hill, and a slide which I have had cut from 

 a specimen taken by Mr. Hill seems to me to accord best with this view ; at 

 any rate it indicates intense crushing. 



