Vol. 68.1 PETKOLOGICAL NOTES ON GUERNSEY, ETC. 49 



The above-mentioned specimen of Pleinmont rock shows a 

 distinctly clastic structure. It consists of rather numerous grains, 

 subangular to fairly rounded (from about '008 inch in diameter 

 downwards), of quartz, with rather wavy edges, and of felspar 

 (plagioclase recognizable) and some dark-brown, barely translucent 

 grains of iron-oxide. These are embedded in a mosaic of a clear 

 material mixed with films of a mica, greenish to colourless, which 

 vary much in size and occasionally are almost equal in length to a 

 diameter of the larger grains. Crossing the nicols shows the clear 

 material to be granular, a mixture, probably, of quartz and reconsti- 

 tuted felspar ; but its individuals are so minute that it is very difficult 

 to be certain (see PI. I, fig. 6). Slices have since been examined 

 from five other specimens collected by Mr. Hill at earlier and later 

 visits. They differ only in details one from another and from that just 

 described, two or three indicating a certain amount of fracture and 

 recementation ; but one specimen (that which shows a slight banding 

 and small fragments) is interesting, as bearing on the relation 

 between the grit and the gneiss. Under the microscope it appears 

 to be distinctly fragmental, some of the quartz-grains being fairly 

 rounded, others rather angular. In most respects it resembles the 

 others, but one part of the slice is considerably finer in grain, and 

 the remainder contains sundry composite grains (those of larger size 

 consisting of quartz and rather elongated felspars, in which plagio- 

 clase is recognizable). One or two of these fragments are only 

 quartz, the constituent grains, four or five in number, being elon- 

 gated and a little ' ragged ' in outline, as is frequent in a gneiss. 

 The altered biotite occurs sometimes in larger grouped flakes, and a 

 small chlorite is present. The specimen fully confirms the general 

 evidence, namely, that the gneiss is anterior to the Pleinmont rock. 



Two specimens of Brioverian rock from Bec-au-Fry (Brittany),, 

 given several years ago by Prof. Barrois to Prof. Bonney, have been 

 sliced for comparison, as they presented a certain resemblance to 

 the Guernsey grit. One, which contains a little epidote and has 

 a slightly more micaceous ground-mass (one fiake measuring about 

 •05 inch), shows a small vein, filled with quartz and some mica^ 

 which is slightly faulted. The other, a little more cracked, is 

 slightly more coarse-grained than the first, and has some of the 

 larger felspar-like fragments replaced by a fairly clear micro- 

 granular mineral, rather like some of the ground-mass. The 

 mineralogical changes in both these and the Pleinmont specimens 

 are rather more conspicuous than is usual in such Cambrian grits 

 as Prof. Bonney has examined, but less than in those of Huronian 

 age near Sudbury (Canada), and in the finer materials of the con- 

 glomerate at Obermittweida (Saxony), and he is disposed to regard 

 it as an effect of old age rather than of contact-metamorphism. 



The three * red dykes ' mentioned above have been examined 

 under the microscope. That above Port Pezeries(A) appears in the 

 hand-specimen to be rather more compact than the ordinary red 



Q. J. G. S. No. 269. ' e 



