Prof. T. G. Bonney—Metamorphism in an Alpine Rock. 509 
schists, which are associated with or rather higher in position than 
the above gneiss. The purplish slaty rock is much less common 
than the other, but it is indubitable, and shows no sign of metamor- 
phism. The matrix is a dark leaden-grey colour, composed of scales 
of mica, mostly silvery, with a variable amount of quartz; it often 
weathers a bright iron-brown; the scales he rather parallel one 
with another, and the whole mass undoubtedly looks extremely like 
a rather tender and somewhat massive mica-schist. The extremely 
sharp boundaries of the fragments however have a suspicious ap- 
pearance, and in order to account for the presence of argillite among 
them, we must credit the metamorphic agents with very selective 
action indeed. 
But the more we examine the rock even in the field, the more we 
become convinced that the metamorphic aspect is illusory, and that 
the mica scales are exogenous and not endogenous. Microscopic 
examination fully confirms this. The rock consists of the following 
minerals—1. Quartz in single subangular grains and granular aggre- 
gates evidently derived from a mica-schist or gneiss. 2. Mica: the 
greater part is a white mica, with silvery lustre, showing brilliant 
chromatic polarization, resembling that common especially in the 
more silvery mica-schists of the Alps; the remainder is a dull brown- 
ish mica, evidently somewhat altered. 5. A granular, somewhat 
earthy-looking mineral, sometimes pretty evidently replacing frag- 
ments of felspar, which is also more or less disseminated among the 
other constituents. Hxamined with the two Nicols it shows a 
peculiar scaly to granular speckling, very familiar to those who have 
investigated rocks of similar nature to this, as an alteration product of 
felspar,’ though it is not easy to assign to it a definite name. There 
is the usual dark dust—ferrite or opacite—and a few grains of a black 
mineral, probably iron peroxide: also one or two grains of a clear 
brown, strongly dichroic mineral, probably tourmaline.? It is 
evident from the structure of the rock that it is of fragmental origin, 
and practically unaltered, only such micro-mineralogical changes 
having taken place as are usual in Paleozoic or even more recent 
rocks. The rock obviously has been greatly compressed, and the 
result of this has been to give a general parallelism to the mica 
flakes and so enhance the resemblance to a foliated rock, but of 
metamorphism in the technical sense of the word there is no trace. 
Below the Pissevache fall, where some of the gneiss is so crushed 
that without microscopic examination it is impossible to say whether 
it has been crushed in situ or is a compressed arkose, we find an 
infold of a flinty greenish rock. The cliffs make examination diffi- 
cult or impossible, and my knowledge of it is mainly derived from 
fallen blocks. It appears to vary from avery compact flinty-looking 
greenish-grey rock, with slight indications of cleavage (rather like 
some of the siliceous argillites or ‘“ hornstones”’ not uncommon in 
the older rocks of Britain), to a more granular and micaceous rock, 
1 Tt may be worth noting that in some remanié rocks the felspar is no longer 
recognizable, in others it is excellently preserved. 
* This mineral has been noticed in the fragments. 
