510 Prof. T. G. Bonney—Metamorphism in an Alpine Rock. 
presenting some resemblance to a remanié gneiss much compressed. 
One great block showed distinct interbanding of the compacter and 
more micaceous varieties, but the former is the more abundant. 
Prof. Favre considers this rock to belong to the Carboniferous series, 
though, as is needful, he expresses himself with caution. I have 
examined microscopically one of the coarsest and one of the finest 
varieties, and though they have no very direct bearing on the main 
questions of this paper, may record the result, as it is rather curious. 
The coarser fragment is so like a crushed gneiss that were it not for 
the label I could not distinguish it. ven under the microscope this 
resemblance is maintained ; so much that I should not have liked to 
express a positive opinion from a single slide. The chief minerals 
are quartz, felspar, orthoclastic and plagioclastic, rather decomposed 
but recognizable, and mica, brown and white, the former in this case 
rather predominating. Not seldom two things are certain: that the 
minerals are associated as in the original gneiss, and that they have 
been broken in situ. I conclude then that we have here a seam made 
up largely of comparatively unrolled gneiss fragments, subsequently 
subjected to enormous pressure which has cracked some and welded 
all together. The structure of the compact flinty “argillite” is 
even more remarkable. JI expected to find the usual minutely 
granular aspect of this rock, specks or fragments of clear quartz in 
a slightly earthy base, full of ill-defined crystallites. On the contrary, 
we see a fairly well-defined fragmental structure, in a minutely 
granular ground-mass into which the fragments seem sometimes to 
melt away. Crossing the Nicols a good deal of the minute filmy 
mineral, often called sericite, becomes visible, with the usual sub- 
parallel rootlet-like arrangement, common in schistose rocks. This 
of course is of the nature of foliation, but it is a change which in 
rocks of proper chemical composition readily takes place under pres- 
sure, and is as far away from the true foliation of a gneiss or mica- 
schist, to use a rough illustration, as the caterpillar is from the 
butterfly. Crossing the Nicols, the sericite becomes more con- 
spicuous, being most highly coloured when the general direction of 
the fibres makes an angle of about 45° with the vibration planes of 
the Nicols, but the clastic aspect of the slide becomes far less marked. 
Many fragments are wholly replaced by a chalcedonic structure, 
speckly-translucent granules interspersed among darker, others are 
traversed by irregular bands showing this structure; the edges of 
most appear to melt through it into the surrounding ground-mass, so 
that not seldom one might reasonably have doubted whether the 
apparent fragments were not rather segregations. 
On inserting a quartz-plate, the original structure is not only 
restored, but rendered far more distinct. It is then evident that 
even this remarkably compact and homogeneous looking rock has 
been made up to a large extent of fragments, sometimes as much 
as 0:06” in diameter, of quartz and felspar derived from a gneiss 
(I do not see any fragmental mica in the slide). This material has 
been subjected to immense pressure, the quartz grains have been 
broken, the felspar crushed; from the latter and from the inter- 
