The lherzolite of Arie

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THE LHERZOLITE OF THE ARIEGE.

[Extracted from the Geological Magazine, February, 1877.]
Trubner & Co., 57 and 59, Ludgate Hill, London.

^Extracted from the Geological Magazine, Decade II., Vol. IV.
No. 2, February, 1877.]

THE LHEEZOLITE OF ARIEGE.

By the Rev. T. G. Bonney, M.A., F.G.S. ;

Fellow and late Tutor of St. John's College, Cambridge.

THE rock Lherzolite has been described by Prof. Zirkel in bis
valuable Beitriige zur Geologiscben Kentniss der Pyreniien
(ZeitscJmft der Deutsch. Geol. Gesel, vol. xix. j). 68), but is generally
passed over with the briefest mention or entirely omitted in English
works on Geology. Even in Cotta's " Eocks Classified and Described "
it is barely noticed, and the word is left out in the index. On this
account, and seeing that, so far as I am aware, no description of its
microscopic structure has yet been published, a notice, embodying
the results of Prof. Zirkel's paper, and of a brief visit of my own to
this not very accessible locality, may be useful to students.

Lherzolite is a crystalline aggregate of the minerals olivine, en-
statite, and diopside, with some picotite, in texture varying from
finely to rather coarsely granular ; that from the locality visited by
myself being, on the whole, of the former character. It obtains its
name from the Etang de Lherz, a small tarn in the Eastern Pyrenees
(Dept. Ariege), above Aulus, in the valley of the Garbet, 38 kil.
from St. Girons, and near the Col d' Erce (or Port de Lherz), an
easy pass (5341') leading to Vicdessos in the valley of the Oriege.
The rock entirely surrounds the Etang, and is the largest of a linear
series of seven exposures in the vicinity of Vicdessos.

The Etang de Lherz is a shallow tarn occupying apparently a true
rock-basin, the longer axis of which lies roughly N. and S. The
water escapes from the northern end by soaking through some peaty
ground. On the western side is a tiny island. The tarn is sur-
rounded by rounded masses (probably once ice- worn) and fallen
blocks of the Lherzolite, which also rises from the western shore in
a craggy hill. A furlong or less from the eastern shore limestone
shows through the grass and stretches away in that direction, forming
the general mass of the country. The tarn is not in the line of the
main valley of the Garbet, but in a sort of open upland glen, a little

2 Rev. T. G. Bonncy—The LherzoUte of the Ariege.

above the bed of the former. On the opposite side of this rises a bare
craggy limestone hill, capping the Lherzolite which forms its base.

The Lherzolite is tough and difficult to break, traversed by many
minute, rather irregular, divisional planes, with occasionally a slight
tendency to a platy structure. Hence it is not easy to obtain good
specimens. The surface of a specimen from the heart of the rock
is rougli, rather uneven and granular, at the first glance tolerably
uniform in colour and apparent composition, of a dark greenish-
grey or olive-green colour. A closer examination shows specks of
brighter green, generally of two colours, one (the more invariable)
an emerald green, the other a waxy-looking duller green ; also specks
of a resinous pale-brown mineral, sometimes with a platy or fibrous
aspect and a dullish lustre ranging from silvery to brassy. Minute
grains of an irregularly disseminated black mineral, with a vitreous
lustre, are also just visible; and there is another of transparent glassy
aspect. The last is only broken olivine, to which the predominant
dull-coloured mineral belongs ; the emerald green is the diopside ;
the resinous mineral enstatite; and the black is picotite. The duller
green tint is serpentine. The separate minerals are more easily
detected in a coarser specimen, which I purchased from Pisani in
Paris in 1875, who obtained it from Sem, the easternmost locality
along this line of outbursts in the Department of Ariege.

The rock at the Etang de Lherz varies a little in texture, some,
especiall}^ as it appeared to me, that towards the outside, being more
compact than the rest. "When the rock is slightly decomposed the
dull green tint becomes more marked, and the compact varieties
begin to resemble serpentine. The exterior weathers from a bright
yellowish to a dark rusty-brown tint, with a rough surface. On this
the projecting pale amber-yellow grains of enstatite, and the bright
green grains of diopside, with the black picotite, may be readily
distinguished. Occasionally also a sort of linear structure is developed
on the surface in weathering ; such as I have observed in some of
the Lizard serpentine ; like this, it has some connexion with au
internal parallelism, but the exact nature of it is not yet quite clear
to me, though I think it will prove to be connected with a fluidal
structure. The brown weathered surface generally extends inwards
for about -1 to -2 inch ; and the change from it to the green rock is
pretty sudden, a thin pale band usually intervening, in which the
enstatite, diopside and picotite are well distinguished. The rock is
traversed by numerous irregular joints, breaking it up into rude
polygonal blocks ; but now and then the outside of an old weathered
surface shows a more regular prismatic structure ; occasionally also
there is a slight parallelism in its fissures. The more minute joints
are lined with a thin fihu of limonite or of a serpentinous mineral,
apparently a green steatite, — often in the latter case so thin as to be
a mere glaze. Slickensides are not rare on the joint faces. The
general asjiect of the weathered rock, the peculiar roughened surface
with its irregular fissures, the jointings and contours of the fallen
blocks, in shape like masses of broken curd, strongly reminded me
of the Lizard serpentine in Cornwall, with which I am very familiar.

Rev. T. G. Bonney — The Lherzolite of the Ariege. 3

Time did not allow me to cross the valley and examine the
junction with the limestone on the opposite side, Avhere it was well
exposed for a considerable distance at the base of a sort of cliff"; but
as far as I could see it was rather wavy and uneven, as if the Lherzo-
lite were intrusive. I followed the junction on the east side of the
pool for a considerable distance. Unfortunately the abundant herbage,
the number of scattered boulders, and the peculiar weathering of
the limestone, which forms deep fissures (like the karrenfelder of the
Alps), harbouring a rich vegetation, prevented me from obtaining a
single actual contact : but as the Lherzolite clearly appears here and
there to protrude in broad tongues into the limestone, and this is
highly crystalline (being quite white and saccharoidal) near the
junction, I have little doubt the rock is intrusive. That it is an
igneous rock I think no one who has examined it will dispute.
There are, however, I think, no proofs of eruption, though a breccia
of angular fragments of Lherzolite and limestone might seem at first
sight to be a volcanic agglomerate, and so even favour the idea of
contemporaneous volcanic action. According to Prof. Zirkel this
breccia occurs here (and here only) between the Lherzolite and the
limestone. I did not, however, observe it at this part of the junc-
tion, but found a dyke-like mass of brecciated Lherzcllte on the
opposite side of the Etang. The numerous fallen blocks made
it difficult to examine this in situ, but it appeared to be about three
or four yards wide, and to cut across the Lherzolite roughly from
E. to W. As far as I observed, however, this rock was com-
posed only of Lherzolite, and I fully believe it only to be a
friction breccia, and not at all of the nature of a volcanic agglome-
rate. The other masses of breccia which I examined were on the
grassy hill-side nearer to the Col d' Erce, not far from where
there is another small patch of Lherzolite on Prof. Zirkel's
sketch-map. These, however, appeared to me to be in every case
erratics, and I could not see the rock in situ on the hill above.
My time, however, was too limited to allow of a long search. These
blocks varied from a breccia of angular and subangiilar fragments of
Lherzolite, frequently more than three inches in diameter, imbedded
in a ferruginous paste which often appears to consist mainly of
minute fragments of Lherzolite, to an extremely pretty rock chiefly
composed of fragments of white marble, often from a half to one
inch diameter, imbedded in a speckly yellowish or greenish grey
matrix, with a slight ruddy tinge. In the time at my disposal I
collected four varieties of the breccia, forming a fairly complete
series. The first is exclusively made up of Lherzolite, and so
thoroughly compacted that (as in many ancient breccias) it is
often not easy to distinguish the fragments, except on a weathered
surface. The second consists mainly of Lherzolite fragments with
a very few small pieces of marble, but here and there there is an
appreciable proportion of minute calcareous fragments in the matrix.
In the third, the marble pi'edominates, but the paste contains a large
quantity of comminuted Lherzolite ; and in the fourth fragments of
marble abound, but those of Lherzolite are rare, though this rock

4 Rev, T. G. Bonney — The LhcrzoUte of the An'ege,

is represented to some extent, as in the last, in the paste. In 'this
(in the last two eases) one can readily distinguish bright green
fragments of diopside and rather numerous black grains of picotite,
apparently imbedded sepai-ately.

This mass of Lherzolite is the largest of the seven exposures in
tlie district, and according to Prof. Zirkel is about 1300 yards in
greatest length. Three other masses lie near it along the line of
the little glen of the Sue. The rest are near its junction Avith the
Oritge, one being on the opposite bank near the village of Sem.
All are in the Liassic rocks, and, except the last, are very near their
junction with the granite, which even here is at no great distance.
It is also more coarsely granular than the rock at the Etaug, and the
breccia is wanting. Lherzolite also occurs near Portet d'Aspet, in
the upper Val longue (Castillon), and on the south side of the Col de
Lurde, in the neighbourhood of Eaux Bonnes. The principal rock
here is a limestone with ophite, i.e. greenstone, near it. I have not
seen any of these.

The rather compact condition of the rock, and the fact that the
olivine is in some specimens rather green, and the diopside a little dull
in colour, while the enstatite does not alwaA'S exhibit its characteristic
tructure, makes it often very hard to distinguish the component
minerals of the sjiecimens from the Etang de Lherz. They are better
seen, however, on a polished surface, and can be separated, as
Zirkel suggests, by treating the pounded rock, first with hydro-
chloric acid, and then boiling it with caustic potass. In my
Pisani si^ecimen from Sem, the minerals are much more easily dis-
tinguished, as is the case also, according to Zirkel, in his specimens from
this locality. The composition of the Pyrenean Lherzolite is according
to an old analysis (Zirkel, p. 140) :— SiO.,=4o-0, A1A=1*0. CaO=
19-5, MgO=lG0, FeO=:12-0, CrO = 0-5, with a trace of MnO
and loss=6-0. T^asaulx gives the analvsis of a Lherzolite from
Norway (Elem. der Petrograph. p. 338) :— SiO„=37-42, Al.,Os=
0-10, MgO=48-22, reO=8 83, MnO=0-17, KiO=0-23, H^O-^-TL

The rock varies slightly in different parts around the Etang,
both in grain and in preservation. I collected specimens chiefly
from near the southern end, and about half-way down the west
side ; the most serpentinous specimens coming from the former.
Mr. S. Allport, to whom I gave a duplicate from Sem, kindly cut
me a beautiful slide from it, and I have had slides (six in all)
cut from three varieties collected by myself at the Etang. I will
refer to them as No. I. (from Sem [Pisani]), No. II. (specimen from
the west side), No. III. (specimen from the south end), No. IV.
(specimen showing a partial passage into serpentine). This speci-
men was cut close to a joint face where the change was greatest.

Microscopic Structure. — In all cases the rock is normally com-
posed of olivine, enstatite, diopside, and picotite, with occasional
minute specks and microlithic aggregates of an opaque black mineral,
probably magnetite. Microliths of other minerals are rare. The
first three minerals all occur in variable shaped grains ; those of
the olivine roundish ; the diopside occasionally showing a slight

Rev. T. G. Bonney — The LherzoUte of the Ariege. 5

approach to a regular crystal outline ; the enstatite usually irregular
aud longish ; the olivine appears to have crystallized the first, but
I think the diflference has not been great. It generally forms about
■| of the whole mass of the rock. The picotite, from its shape, seems
to have crystallized last.

Tlie olivine occurs in more or less rounded, transparent, colourless
grains, very irregular in size. Surface finely granular, something
like frosted glass. Colours with crossed Xicols often very beautiful,
commonest from a translucent greenish yellow to a yellowish green,
and from a bright to a purplish pink. Owing to the peculiar texture,
one of these tints often overspreads the other something like a shot
silk. The mineral shows the usual rather irregular cracks, indi-
cating its imperfect cleavage. These often cause, by imperfect
cohesion, colour bands, which are also common near the edges of
the grains. Not seldom we find in the olivine small vermicular
cavities arranged in slightly wavy bands. These appear to be
sometimes empty, sometimes filled with a brownish mineral, perhaps
iron peroxide. They lie in some cases in the planes of imperfect
cohesion, and then have often a dendritic character. There are
occasional clots of an opaque dust-like mineral, probably magnetite,
and thin fibrous brown films, strongly dichroic, which may either be
mere stains or minute plates of iron-glance. The last are often
associated with the picotite.

The enstatite is transparent, colourless in ordinary light, with a finely
granular or slightly silky texture. The cleavage parallel to ooPoo
is generally well exhibited, though not so close as a rule as in.
diallage ; a more interrupted cleavage parallel to aJr is also some-
times fairly distinct, as in Rosenbusch, Mikroscop. Physiogr. Tab.
viii, 44. In cases where the specimens have a less characteristic
aspect, I have found the principal cleavage planes better exhibited
by rotating the microscope stage till the plane of the principal cleavage
is nearly parallel to the plane of vibration of one of the crossed Nicols,
when, as the cr3'stal approaches its darkest aspect, the fine cleavage
becomes more clearly visible. This method (proposed by Tschermak)
of distinguishing the orthorhombic enstatite from the monoclinic
diallage will be found very useful in examining Lherzolite. The
crystals show sometimes wavy bands crossing roughly at riglit
angles the lines of the principal cleavage, formed apparently by
minute elongated cavities aud microliths. Colours with polarized
light pale yellowish or greyish to various blues.

The diopside is not generally in well-formed crystals ; it is pellucid
in the thin slices, and sometimes still retains a faint tinge of green.
With polarized light, the colours are less diaphanous in aspect than
those of the olivine, rich yellowish-brown and puce tints being
common. The surface is rather variable, but generally moderately
rough-looking, with often a slightly " stepped " aspect. The
characteristic cleavage, as in augite, is commonly well developed.

The picotite occurs in very irregular grains or groups of grains, or
even films, often looking as if a point armed with a sticky fluid
had been drawn for a short distance along the slice. Surface rather

6 Eev. T. G. Bomwj—The LherzoUte of the Ariege.

rougli-looking, something like that of augite. Colour a translucent
rather deep olive green, occasionally slightly inclining to brown,
in No. IV. a rich umber brown. Eosenbusch (Mikroscop. Physiog.
p. 160) gives the colours of picotite as yellow to brown, transparent
to opaque ; stating that Pleonaste ditfers from it in having green
tints. If this distinction be correct, the mineral in slides I., II., III.
must be Pleonaste. The grains are traversed by rather irregular
cracks, which occasionally indicate a rude cleavage. IV. is less
rich in picotite than the rest. As the mineral is isometric, it is of
course dark between crossed prisms.

Of the various slides, No. I. is the best for study of the rock, as
it is more coarsely crystalline, and shows little or no indication
of decomposition. No. II. shows the grains of the minerals a little
more rounded than No. I., and all are much cracked. The olivine
appears to bear a rather smaller proportion to the other minerals
than in I., and the diopside shows a rather smoother texture.
The cracks in the olivine are often bordered on both sides by
a finely fibrous serpentine, the result of decomposition. It remains
bright, generally of a pale golden hue, between crossed prisms.
No. III. is in structure similar to II., but with more olivine ; here
decomposition has advanced further, giving parts of the slide a
muddy look, probably due to faint stains of peroxide of iron ; the
serpentine strings are often abundant enough to fonn a kind of net-
work in the olivine, and one considerable crack across the slide
is filled by a feebly double refracting serpentinous mineral. There is
a sort of parallel structure perceptible in the direction of the
principal cracks, marking a parallelism in the axes of the crystals,
and the same is to a slight extent perceptible in the arrangement
of the minerals.

No. IV. gives indications of a structure similar to III., but the
change here is much more considerable. A. network of serpen-
tinous strings covers almost the whole slide, in many cases invading
the other minerals ; the cracks of which are usually free from
serpentine in II. and III. In parts the strings seem to coalesce, so
as to convert appreciable portions of the slide into serpentine.
Here it is interesting to note that clots of opaque dust, doubtless
oxides of iron, resulting from the separation of the constituents of
the olivine, appear among the strings just as we see them, for
example, in the Lizard serpentines.

These slides therefore exhibit to us, and this is the most interest-
ing asjject of the rock, the commencement of the formation of
ser[)entine. In certain serpentines — as, for example, those of Elba,
and, as I have recently discovered, of the Lizard — and in some of
the olivine bearing gabbros, we can trace the process from specimens
from which all the olivine has disappeared, and the alteration into
serpentine is complete, to those in which a considerable amount of
unchanged olivine is still to be detected. "We have thus a further
confirmation of the idea, now becoming not unfamiliar to geologists,
that much serpentine is an altered olivine rock.

Stphen Austin and Sons, Printers, Hertford.