measurement. Nevertheless, the chemical properties, and comparison with numerous authentic specimens of chondrodite, leave no doubt concerning the accuracy of our determination.

Although rarely transformed completely, the chondrodite presents numerous and interesting alterations. It is often broken, and in each of the fissures there has been produced a thin vein of chrysotile, which sometimes entirely surrounds the crystal and separates it from the enveloping calcite. More frequently there is a zone, of about a tenth of a millimetre, around the crystal, in which the chondrodite is reduced to fine débris that remains orientated in the same way as the crystal from which it is derived, but cemented by the calcite: the latter often occurs in extremely fine particles.

In such cases, between this zone of partial alteration and the dolomite in large plates, there is a 'muff' of calcite granules placed side by side tangentially to the crystal of chondrodite, in such a way that their principal axes are normal to the surface of the crystalline grains that they surround. (Fig. 15.)

The pyrrhotine occurs in the form of little hexagonal prisms ooP.OP.(1010) (0001). They did not escape de Bournon, who noticed their magnetism, and regarded them as a martial pyrites' containing less [sulphur] than usual.

At Cornigal, about 40 miles (16 lieues) N.-W. from Colombo, cipolins are found, composed in great part of yellow calcite, and enclosing elliptical masses up to a foot (3 decimetres) across. These are made up of a mixture of different minerals, and are very variable in constitution. Two of them were found to have the following composition:

1st. The first is composed of large crystals. With the naked eye, calcite and a white felspar in large plates, green pyroxene, pyrites and quartz, can be distinguished. The examination of microscopic sections shows that the pyroxene belongs to the scarcely pleochroic sea-green variety described above.

All the constituents are granular. Besides those mentioned sphene is present in strongly pleochroic, bright pink crystals. The felspar is oligoclase. The calcite forms rounded grains, playing the same part as the other minerals: it also occurs in little patches due to secondary infiltration.

2nd.-Seen with the naked eye, the second is green, there being visible green hornblende, calcite, a white mineral not attacked by acids, and pyrite.

The microscope further discloses abundance of wernerite in large patches, sphene, oligoclase and zoisite.

The hornblende is of a pale green colour, with a rather strong pleochroism—

with b>c>a.

c, bright sea-green,

b, brownish-green,

a, bright-yellow,

Extinction on ooPoo (010) occurs at about 22° from the direction of elongation. The crystals are greatly elongated in the direction of the edge of the zone ∞P∞.00F∞.(100)(010), and often form long needles placed irregularly side by side, and separated by patches of calcite.

As in the preceding rock, the calcite exists in two different states, secondary

1 Op. cit., p. 310.

2 Sic: doubtless Kurnegalle, about 50 miles N.-E. of Colombo. Vide footnote, p. 9—F.R.M.

calcite being here very abundant. These lenticular nodules of variable composition recall those that Becke has described in the pyroxenic gneiss of Seybererberg, near Weissenkirchen1

In the present case, they are of great theoretical interest, for being evidently derived from the cipolins in which they occur, they present a composition closely comparable to that of the pyroxenic gneisses of the same region, and, in a certain measure, allow us to assume, for the latter, a similar origin.

MICA-SCHISTS.

The upper part of the gneissose series is composed, in the neighbourhood of Salem, of hornblendic mica-schists, quartzose rocks containing iron-ore (magnetite), and a peculiar hornblende (grünerite), mica-schists with chromiferous mica (fuchsite) rocks, containing ripidolite and clinochlore, and finally chloritic schists and sericite-schists, which are very like the archæan schists.

We will rapidly pass in review those of them, and those only, which are of petrographical interest.

Hornblendic mica-schists.

These mica-schists are confusedly stratified rocks, formed of little needles of olivegreen hornblende. The rock is easily broken, when these little crystals become separated.

In microscopic sections they are transparent, slightly coloured and pleochroic with:

The rock contains a considerable quantity of magnetite in rounded grains.

Mica-schist with grünerite.

These rocks have an extremely well-marked stratification. Sometimes they are rich in quartz; sometimes, on the other hand, they contain none, and are formed exclusively either of hornblende (grünerite), or of the same hornblende with magnetite. They were found 12 miles (5 lieues) west of Salem.

Usually the rock is strongly impregnated with oxide of iron, and has a reddish colour; when this tint is less marked, the rock is brownish yellow. When it is composed exclusively of hornblende, it occurs in the form of but slightly coherent, large fibrous masses; when including quartz and magnetite, it becomes, on the contrary, very tenacious.

By treatment for some minutes with boiling hydrochloric acid, all the products of infiltration may be dissolved, and the hornblende appears in yellowish olive-green transparent crystals of 2 or 3 centimetres, resembling the hypersthene of the rocks of the Capuchin (Mont-Dore), but having only an extremely feeble pleochroism, with a maximum absorption parallel to c.

'Tsch. min. petr. Mittheil., iv (1882), 374.

The microscope shows that this hornblende, which is much elongated in the direction of the edge ooPoo.00Poo.(100) (010), does not possess a definite form. The extremities of the crystals are irregular: the cleavages P.P. (110)(110) are very clearly marked, and also the [gliding planes] parallel to oP.(001).

In sections normal to the vertical axis, the two prismatic cleavages may be observed to form an angle of about 124°.

Polysynthetic twinning on ocPoo. (Ico) is very common. Extinction on ∞ Poo. (010) occurs at 25° from the trace of the prismatic cleavages. The double refraction is very strong:

All these characters are similar to those that I have noticed in the exclusively ferriferous variety of hornblende found in the mica-schists of Collobrières (Var) associated with ores of iron.

The reddish colour of this hornblende, when examined in large specimens, is due to a sort of envelope of hematite, which coats each crystal, and which can be removed by acids, as has been said above. The hornblende is fusible before the blow-pipe to a black magnetic glass: its chemical composition is the same as that of the grünerite of Var.

The rock is formed, besides, of magnetite without definite form, and of quartz in grains of about half a millimetre with very distinct outlines, which are often emphasized by ferruginous infiltrations. The foliation of the rock is largely dependent on the orientation of the hornblende.

Mica-schist with fuchsite.

This mica-schist was obtained by Leschenault from the Kaveri, about 40 miles (17 lieues) west of Salem. It belongs to the same geological horizon as the rock just described, from which it only differs in the substitution of mica for hornblende and magnetite.

It is a strongly schistose rock, composed in great part of white translucent quartz, sometimes tinted green by inclusions of emerald-green mica, which is localized in layers, the foliation of the rock being thus produced. Certain varieties, in which the green mica is very abundant, are susceptible of a fine polish.

The laminæ are hexagonal, but always more or less jagged.
The pleochroism is well marked, even in microscopic sections :-

f} bright-green.

a, colourless.

The colour is of a fine emerald-green, in thick plates.

The plane of the optic axes is perpendicular to ooPoo.(010):

2 E = about 55.

In respect to its properties this mica is very near the fuchsite of Zillerthal: before the blow-pipe it gives the re-actions of chromium.

1

The great mass of the rock is composed of a little oligoclase, and much quartz.

The two chloritic rocks mentioned above are formed, one of a fine green

À Caveri." The river, where due west of Salem, is about 30 miles distant.— F.R.M.

clinochlore in plates 2 centimetres in diameter (2 E = 45° across the positive bisectrix); the other of a granular ripidolite (1 positive axis), which is strongly pleochroic in microscopic sections

e, colourless.

o, green.

This rock is remarkable from its analogy with those found in certain parts of the Alps, and notably at Zermatt. Like the Zermatt rock, it encloses crystals O.(111) of magnetite which attain nearly a centimetre in their longest direction.

CHAPTER VIII.-SUMMARY AND CONCLUSIONS.

The title of this memoir itself shows sufficiently that we have had no intention of writing a complete monograph on the pyroxenic gneisses and scapolite-bearing rocks. We have only desired to lay the foundation of a work that we propose to carry out.

The great abundance of scapolitic gneisses is shown in the first line of our memoir, and we are persuaded that this category of rocks that have been little studied up to the present time will be found in most regions where the upper series of the gneisses exists.

From the descriptions given in the preceding chapters it is possible to draw some general conclusions.

In the upper part of the series (l'étage) 1 of the gneisses, pyroxenic rocks occur which are characterized by a special facies. They never constitute beds of a great thickness that can be followed for long distances. They form lenticular masses, elongated in a fusiform way, and often disposed one after another in lines (entraînées). They are conformable in stratification with the acid gneisses of the series under discussion, and follow all the contortions and deformations of the latter.

Sometimes, as in Finisterre, Waldviertel, &c., they are associated with hornblendic gneisses, often garnetiferous (eclogites), forming alternations of various rocks ad infinitum, and passing into each other by insensible gradations. Sometimes (Norway and Saxony) they are associated directly with granulitic gneisses, presenting numerous passages into these more acid rocks.

Viewed with reference to their mineralogical composition, these pyroxenic gneisses are characterized by the association of minerals, small in number, but varying considerably, both in composition and proportion, not only in different regions, but in the same locality.

They are rocks with a compact appearance, and colour varying from greenish gray (Brittany) to dull green (Saxony). They are very tenacious, and in general finely crystalline. The varieties passing into hornblendic gneisses are the darkest. Those passing into granulitic gneisses (Saxony, Odegarden), by the introduction of quartz and orthoclase, are of a bright colour; their constituents are easily distinguished with the naked eye, and the general facies of the rock recalls that of granulitic gneiss.

As a rule, these gneisses are not foliated; their crystallo-laminated (cristallophyllienne) nature is indicated by the distribution of the coloured minerals in parallel planes. Except in some special cases this disposition is more apparent under the microscope than to the naked eye.

The most simple composition is found in a granular mixture of triclinic felspar, or scapolite, with pyroxene. Sphene is rarely absent (Saxony), and is sometimes abundant. Pyrrhotine is of common occurrence.

very

Next to the above, hornblende, in the varieties passing into hornblendic gneisses, quartz, apatite, zircon, garnet, hypersthene, wollastonite and calcite are the most frequent constituents. As for the felspars, anorthite (Roguédas, Ceylon), labradorite (Brittany, Saxony, &c.), oligoclase, (Brittany, Saxony, Waldviertel, Norway), albite(New York) or orthoclase (Brittany), may be present, either singly or more than one together, even in different parts of the same bed.

The minerals of the scapolite group play the same part. Divers types of the family (dipyre, wernerite, &c.,) are met with in these rocks, as will be seen further on, but, conversely to what is true for the felspars, one species of scapolite is characteristic of any given locality (dipyre in Brittany, Spain and Pierrepont, wernerite at Odegarden, Waldviertel, Ceylon, &c).

The relative proportions of the white constituents, and of the ferro-magnesian minerals, are not less variable; sometimes, as in Ceylon, and occasionally in Brittany, the white minerals form nine-tenths of the rock, enclosing a few grains only of pyroxene; sometimes, on the other hand (United States), the pyroxene forms exceptional rocks by itself.

The pyroxene appertains to malacolite; it is of a bright green colour, rarely very pleochroic (Saxony). In two localities (Ceylon and Odegarden) the scapolite s accompanied by peculiar, intense-green pyroxenes.

Wollastonite occurs either as an original constituent (Ceylon, Herero, and the United States), or as a product of the decomposition of basic felspars. Allanite, crystallographically allied to epidote, is frequent in these rocks (Finisterre, Norway, Waldviertel). Epidote is a primary constituent of them. In the anorthite gneiss of Salem we have discovered a new species of mineral-fouquéite.

Turning to the hornblendic gneisses accompanying the above pyroxenic gneisses, they offer still more remarkable examples of variations in composition. We have examined them incidentally, and have been obliged in studying them to adopt a special classification for each locality, generally based on the nature of the dominant hornblende. The pegmatoidal associations of bisilicates, either in the mass of the rock, or in coronas around the garnets, may be observed in all the localities enumerated above.

We have seen that in Norway there is a scapolite-bearing-rock independent of the gneisses, and that it was produced by the metamorphism of gabbro under the influence of veins of apatite. The study of this ophitic gabbro has led us to the examination of a French exposure of the rock that we have discovered, and in which we have found petrographical peculiarities (coronas around olivine) similar to those of the Scandinavian gabbros. Finally, still another scapolite-bearing-rock has been described. It is an eruptive one, of which the history has not been completely elucidated.

From a genetic point of view, it is interesting to remark that in divers localities

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