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PEOr. T. G. BOXXEY OJT THE 



pit about 5| kilometres from the town is very slightly west of north. 

 Under the microscope the foliated structure is distinctly indicated 

 by wavy bands running across the slide, like rootlets growing in 

 the direction of the lines of foliation. These are occupied by a 

 mixture of white mica, often microlithic, tiny scales of brown mica 

 (occasionally), an earthy-looking mineral, and chalcedonic quartz. 

 Between these bands occur fragments of felspar, an irregular mosaic 

 of quartz and felspar, and larger scales of mica. Among the felspar 

 fragments, which are sometimes a good deal decomposed, orthoclase 

 can, I think, be recognized, also microcline, and a little plagioclase 

 with lamellar twinning. In the most decomposed felspar tiny scales 

 of white mica have often formed. It is rare to find a quartz grain so 

 much as 0*02 inch in diameter, but lenticular aggregates composed 

 of irregularly outlined and often flatten ed-looking granules, just like 

 the structure in a mica-schist, are frequent ; sometimes the majority 

 of the granules in one of these aggregates show the same tint. The 

 brown mica also often occurs in aggregated flakes, and I am inclined 

 to refer some associated white mica enclosing black flakes to an 

 alteration of biotite. It is to my mind obvious that we have here 

 a case of the formation of a gneissoid rock from a fairly coarse 

 granitoid rock b} T crushing, followed by a certain amount of mineral 

 rearrangement — i. e., a case of cleavage-foliation. My impression 

 in the field, and it is not contradicted by microscopic examination, 

 was that this gneissic rock had been developed from the granite 

 already described. 



About one kilometre further is a large pit in a very different rock, 

 the lialleflinte of Barrois. Two varieties of it occur here — the one, 

 black, compact, with sharp subconchoidal fracture, behaving under 

 the hammer almost like flint ; the other slightly schistose and 

 streaky, rather grey in colour, and exhibiting an " augen-structure 99 

 on a small scale. Under the hammer the latter is more fissile, and 

 does not fly into such sharp-edged chips. The banding is about 

 vertical, striking between E.N.E. and E. Both rocks weather a 

 pale grey colour, and are much jointed. In the time at my disposal 

 I could not find any distinct line of junction between the two 

 varieties, but in the pit the grey rock certainly occurs on both sides 

 of the black. On microscopical examination the grey rock proves, 

 as I had already suspected, to be a rhyolite, in which subsequent 

 pressure has produced a slight schistosity. We can still recognize 

 the remains of a flow-structure; the "eyes" are porphyritic crystals 

 of felspar ; sometimes they are almost crushed, sometimes they have 

 been sheared, and the one fragment pushed in advance and slightly 

 separated from the other. One crystal, now about *15 inch long, 

 has had its ends crushed, and the other part split and sheared, so 

 that the pieces rest one on the other like a series of slabs placed on 

 the slope. The rock does not appear to have formerly contained 

 any large quartz-grains, but two or three flakes of white mica 

 resemble an original constituent. The crushing appears to have 

 acted nearly at right angles to the actual flow-structure. It has 

 developed, as usual, a minute filmy golden-coloured mineral (seri- 



