208 Prof. T. G. Bonney—Quarts-Schists from the Alps. 
are a little lenticular in outline, as if slightly nipped. In a specimen 
from the south side of the Lukmannier Pass, where thin laminz of 
mica alternate repeatedly with layers of quartz, and the rock macro- 
scopically shows beautiful flexures, it is clear that the folding took 
place after the foliation had been produced. The grains of felspar 
are often a little larger than the quartz, but of the same general 
form. Asarule they are more or less decomposed, but not seldom 
are in fair preservation, showing characteristic twinning, ete. 
In regard to the other specimens collected from the Mittaghorn ; 
two were taken respectively 6 feet and 10 feet from the one already 
mentioned ; the second near to the seeming passage of the quartz- 
schist into the gneiss. They are more micaceous than the former one, 
and the first is rather gneissic in aspect, as if some felspar were 
present. On microscopic examination that proves to be the case. 
The felspar grains, far from numerous, are irregular in outline like 
the quartz, and are rather decomposed. Two or three of them are 
about ‘07” in diameter, while the quartz grains seldom exceed and 
are often less than ‘01”. The second specimen only differs from that 
already described in being distinctly more micaceous, and in having 
its quartz grains a shade smaller. In this but little felspar is 
recognizable though a disseminated brown dust gives the slide a 
less clear aspect. A fourth specimen was taken at a distance of 
about 100 yards, very close to the junction with the gneiss; the 
precise line of demarcation, as already said, being impossible to 
determine.t Here the constituents are decidedly more variable in 
size. Much of the mica (abundant) is about as large as that in the 
other slides, but there are many flakes barely -01” long. The quartz 
is occasionally about the same diameter, but as a rule is much 
smaller. Very little felspar can be recognized, but occasional clouds 
of brown dust suggest that it may have been present, and the slide 
as a whole is dirty.” 
Next as to the relation between the present structure and the 
pressure which these rocks have undoubtedly undergone. In the 
cases under immediate consideration, the fissility and the foliation are 
parallel to the banded structure, so the modifying pressure must 
have acted at right angles to it. The constituents of the rock do 
not suggest shearing forces of importance, nor indeed so much 
‘flattening out” as one would anticipate. In some cases no doubt 
a peculiar clotting and mixing of grains and granules may indicate 
something of the “kind: but in these specimens from the Saas-thal 
only a moderate amount of mechanical disturbance is suggested. 
The condition of schists which belong to the same group and occur 
as fragments in the Rauchwacke (Trias) indicates that the foliation 
was anterior to the Tertiary mountain-making, so that in all 
probability these rocks had very nearly arrived at their present 
1 Thus labelled in the field: ‘seems to be the base of the quartz-schist, just. 
against wrinkled gneiss.’ 
"2 Tn these slides a few rather irregular grains of a clear mineral are present, which 
is probably zoisite, and in one garnet * (one or two grains of rather fragmental 
aspect) is probably present. 
