Oct. 8, 1885] 
NATURE 
Deis 
mens described in this communication were collected by Mr. J. | because a rock which is now, both macroscopically and micro- 
G. Buchanan, during the voyage of the Challenger. The islands 
have been described by Darwin in his ‘‘ Geological Observa- 
tions on Volcanic Islands” (2nd edition, p. 27). 
after having explained the geological structure, gives lithological 
descriptions of the chief types of the rocks, which may be re- 
ferred to the phonolites (St. Michael’s Mount). These pho- 
nolites are composed of sanidine, augite, nepheline, hornblende, 
magnetite, nosean, and titanite. The rocks of Rat Island are 
basalts with nepheline. The constituent minerals are augite 
and olivine. The ground-mass is almost entirely composed of 
nepheline. Biotite and apatite occur as accessory constituents. 
The little island known as Platform Island is also basaltic, with 
a doleritic texture. It is composed of labradorite, augite, 
olivine, magnetite, and biotite. This rock has undergone 
alterations. 
Preliminary Note on some Traverses of the Crystalline District 
of the Cental Alps, by Prof. T. G. Bonney, D.Sc. LL.D., 
F.R.S., Pres. G.S.—During the past four years I have made 
several traverses of the Central Alps from north to south, and 
venture to lay before the Section the general results as bearing 
in some respect on the geology of the Highlands. (1) The 
ordinary rules of stratigraphy as learnt from most lowland 
districts are commonly quite inapplicable to the Alps. The 
most highly crystalline and the older beds often form the higher 
parts of a mountain region, the newer the lower. The newer 
beds frequently appear to underlie and dip regularly beneath the 
older. Gigantic folds, overturns, and overthrust faults abound. 
The true stratigraphy of a district can only be worked out by the 
exercise of patient and cautious induction from observations 
extended over a wide area. (2) The non-crystalline rocks of the 
Alps are of various ages. There are some of Carboniferous age, 
but the great period of continuous deposition generally begins 
with some part of the Trias. The conglomerates, which often 
occur at the base of the non-crystalline deposits, indicate that 
the principal metamorphism of the crystalline series was anterior 
to both these epochs, There is at present no reason to suppose 
that either in the Central Alps or for some distance on each side 
are there any representatives of the earlier Paleeozoics. I believe 
that the conglomerates at the base of the Carboniferous contain 
fragments of the later crystalline rocks of the Alps as well as of 
some of the earlier—though I do not assert that these crystalline 
rocks have undergone no modifications since Carboniferous 
times. (3) In the heart of the principal Alpine chains, and 
apparently at the base of everything, are coarsely crystalline 
gneisses. These differ little from granites, except that they 
generally—almost always—exhibit a certain foliation, and occa- 
sionally seem to be interbedded with thin seams of micaceous 
schists or flaggy fine-grained beds. (4) On examination we find 
reason to believe that both the latter are generally due to crushing. 
Their strike agrees with that of the apparent foliation in these 
older rocks, and with that of a foliation which is also present in 
the newer crystalline rocks. This corresponds with the strike 
of the main physical features of the district, and with the cleay- 
age in the included troughs of sedimentary rock. It runs for 
great distances with remarkable uniformity. (5) This ap- 
parent foliation is due to the devolopment of extremely thin 
films of a micaceous mineral. In many cases it causes the 
rock to bear the aspect of a highly micaceous schist ; yet, on 
examining a transverse section, the rock is distinctly seen to be a 
crushed gneiss—?.¢. though so conspicuous, it is a mere varnish. 
As it thus differs materially from a true foliation, it would 
be convenient to give it a name, and I should propose to call it 
the ‘‘sheen surface.” It is, in fact, a kind of ‘‘cleavage folia- 
tion,” that is, a foliation due to cleavage, and subsequent to it. 
(6) The pressure which has produced this ‘‘sheen surface” 
has in many cases affected the orientation of the minerals, 
which are present in the true ‘‘foliation” layers ot the 
more distinctly foliated, z.e. mineral-banded, rocks. (7) In 
the crystalline schists very commonly the ‘‘ sheen sur- 
face” corresponds with the original foliation surface, as in the 
slates the cleavage sometimes does with the bedding. This is 
due to the fact that the axes of the great folds often make a very 
high angle with the horizon. (8) Thus a non-foliated crystalline 
rock may be rendered to some extent foliated by pressure (fol- 
lowed by a certain amount of mineralisation): z.e. some gneisses 
may be formed by crushing from granites, some schists out of other 
igneous rocks. It may obliterate an earlier foliation, or it may 
intensify it, or it may produce an independent and more fissile 
foliation. In this sense gneiss may be said to pass into granite, 
The author, 
scopically, a gneiss may prove to be a granite which has in some 
parts yielded to pressure more than in others. (9) As we pass 
outwards from the great central granitoid masses we come to 
gneisses and schists where the evidence of some kind of stratifi- 
cation becomes more marked ; bands of crystalline limestone, 
quartzite, and granulite being associated with mica schist of many 
kinds—simple, garnetiferous, staurolitic, actinolitic, and the like— 
the bands of different mineral character and composition varying 
from mere streaks to layers up to many yards in thickness. In 
fact the above-named rocks are associated exactly as limestones, 
sandstones, and clays are associated in the ordinary sedimentaries. 
(10) Although the crushing of a crystalline rock z# sit, or the 
squeezing and shearing of a breccia or conglomerate of crystall- 
ine fragments, occasionally gives rise to local difficulties, these 
are on a small scale, and sedimentary beds belonging to the 
Palzeozoic or later periods of deposition are generally readily 
distinguishable from the whole of the crystalline series. 
Though folded and faulted in the most extraordinary manner, 
the members of the two series can generally be separated 
and in the Alps there is no evidence of a mingling of the one 
with the other in the process of rolling out or squeezing to- 
gether ; so that, after patient study and microscopical examina- 
tion, we can generally decide without hesitation whether a 
particular set of rocks has originated from the crystalline or the 
sedimentary series. I do not say that we can always decide 
whether a schist or a gneiss has originated from an igneous rock 
or from an older schist or gneiss, but I think that in the Alps 
we can say that it has originated from one of these. Fortunately, 
intrusive rocks are very rare in the Paleozoic and later deposits in 
this part of the Alps. (11) Thus, although the Tertiary meta- 
morphism of the Alpine rocks is very important, it is more pre- 
tentious than real, and its effects seem to have been the greatest 
where it has found a rock already crystalline to act upon. 
Hence I believe that every true gneiss and schist in the Alps is 
much older than the Carboniferous, and is probably older than 
any member of the Palaeozoic period. 
The Direction of Glaciation as ascertain:d by the Form of the 
Strie, by Prof. H. Carvill Lewis.—As there seemed to be a dis- 
agreement between certain Scotch geologists and the Irish 
geologists regarding the inferences as to direction of glaciation 
to be deduced from the form of glacial strize, the author was led 
to bring forward some observations of his own, made in America 
and in Great Britain, which threw light upon the disputed point. 
Well-preserved strice are frequently blunt at one end and taper- 
ing at the other, the shorter ones sometimes resembling the 
characters used in the cuneiform inscriptions. This form may 
be seen in strize of all sizes—from those several yards in length, 
when the blunt end may be an inch or more in breadth, to the 
finest scratches, where a microscope is necessary to detect any 
difference between the two ends. As shown in the Reports of 
the Boulder Committee of the Royal Society of Edinburgh 
(Fifth Report, pp. 18-20, 29, 58 ; Seventh Report, p. 18) and 
elsewhere, certain Scotch geologists regard the blunt end as the 
point of impact of the striating agent, and as therefore facing 
the direction from which the motion came. On the other 
hand the Irish geologists (‘* Memoirs of the Geological Survey of 
Ireland,” Explanation to sheets 86, 87, 88, p. 55; Explanation 
to sheet 193, p. 18, &c.) interpret the shape of the strice as in- 
dicating motion in the opposite direction, believing the tapering 
end to point to the direction from which glaciation proceeded. 
The point at issue is of importance, especially in outlying islands 
and elsewhere, where other indications of the direction of glacia- 
tion fail. In Pennsylvania, which is crossed from east to west 
by the terminal moraine of the great ice-sheet, and where the 
glaciation is uniformly in a southward direction, the author had 
observed that the blunt ends of the strice, where flat surfaces 
were studied, were always io the south (‘‘On the Terminal 
Moraine in Pennsylvania and Western New York.” Report Z, 
Second Geological Survey of Pennsylvania, pp. 33, 85, 86, 107, 
275). Incertain instances the mode of formation of the striz 
was also indicated by their shapes, which showed that a stone 
pushed along under the glacier had ground in deeper and deeper 
until in some cases it stopped or hopped out, in other cascs was 
ground down to another cutting edge, and in others /urned over, 
and began its work of engraving by a fresh and sharp corner. 
The peculiar gorges at the farther end of certain strize showed a 
sort of slow rocking motion in some stones before they finally 
turned over. The author’s observations in Ireland, both at 
localities where there could be no doubt as to the direction of 
