HEN we find at the sides of veins, the veinstone rent into

Wtamine, as I tried to represent in Plate XX. of Vol. IV. it is

easy to think of the fracture as violent, and of the disruption of the vein as sudden.

That, at least, this disruption must have been exceedingly slow, and that as it took place the rent must have been filled by contemporary crystallization, is I think evident in the instances figured, and in the great number of cases which they represent.

And as I continue my inquiry, it becomes more and more questionable to me whether there has in such cases been disruption at all. For the more I endeavour to read Nature patiently, the more I find that she is always trying to deceive us while we are impatient, by pretending to do things in ways in which they never were done, and making things look like one another, which have no connection with

each other.

For instance, in Fig. 1, which

rudely sketches a piece of Cornish M
hornstone, it would seem at first
sight that the detached black and
white bands were pieces of a band
once continuous, but which had
been broken up, and re-cemented
in disorder. And if, on a large
scale, we had met with the fault
in almost exactly coincident beds,
to which the arrow points, we
should have had little doubt of

FIG. 1.

their former continuity. But in this stone they have never been in any other than their existing position, any more than the two upper beds on the left, of which one is an entirely undisturbed branch

1 For former papers see GEOL. MAG., 1867, Vol. IV. pp 337 and 481; also 1868, Vol. V. pp. 12, 156, and 208.

VOL. VI.-NO. LXVI.

34

of the other, as much as any branch of stalactitic chalcedony is of the rest of the mass Nor have any of these beds ever been broken at all. The whole is a tranquil determination of variously crystallizing substances, like that of the component minerals in granite. The white portions are hornstone; the black band in each is ferruginous, and the enclosing paste rudely crystalline quartz.

There is, however, one grave structural difference between this stone and common granite. The crystals in granite run in all directions. These zones of hornstone have a more or less parallel direction; and the black band, with another narrow one succeeding it, is always at the same side of them.

I have placed the woodcut (Fig. 1) with the black beds uppermost, so that the resemblance may be seen between them, and the always uppermost grey beds in the highest division of Plate XV. Vol. IV. But in neither case can I say that their position has been influenced by gravity. For in Plate XV. it will be observed that the elliptical bar of central calcite crystallizes in every direction, and in this piece of hornstone, very near the portion above figured, is a cavity, in which while the bands whose separation forms it, retain their relation unchanged, the quartz, having now room to crystallize, does so indifferently up and down, and from both sides, as in Fig. 2. I do not know the position of the stone in situ.

But though common granites show only arbitrary positions of crystals, in graphic granites we have a definitely parallel arrangement of them, somewhat resembling this of the hornstone, only more regular; and in massive felspathic rock we get the same deceptive resemblance of faults exquisitely defined. Fig. 3 repre

sents (of the real size: as are also Figs. 1 and 2) a portion of felspathic rock in which two crystals of labradorite are separated by apparent breccia, but really, crystalline mass, of mixed labradorite and hyperstein. The oblique lines stand for this gangue (merely for a symbol there are no lines nor cleavage in the gangue itself). The white spaces are pale blue labradorite, the horizontal lines indicate in each crystal a sharp, exquisitely defined, zone of vivid

orange, and the vertical lines a zone of intense blue. There has evidently been no fracture in this case, any more than between the felspar crystals of common granite. And the-in this instance absolutely accurate-coincidence of direction in the zones of the detached pieces, with their fault-like variation in breadth and relative position, are both of them entirely crystalline phenomena.

Now we must always remember that in chalcedony and quartz we have two entirely distinct groups of crystalline forces; one radiant, endeavouring to throw the mass into spherical concretions; the other rectilinear, endeavouring to reduce it to hexagonal crystals and that both of these are capable of producing phenomena of relative distortion.

:

Also, the group of the spheric forces associates itself delightedly with the spheric forces of hydrous oxide of iron, thus producing endlessly fantastic groups of mixed iron and chalcedony, while the rectilinear forces ally themselves in like manner to those of micaceous iron, bournonite, heavy spar, and calcite, producing tabular groups of crystals which present close analogies to the flat leaves of chalcedonies which, have metallic or earthy laminæ for their support; while the iron-oxide, when it has no longer the power of modifying the shapes of the crystals, sets itself to imitate two other minerals frequently found in them. It mimics the globes of brown mica so exactly with its own bossy groups of clustered laminæ, that only a strong lens, or the knife, will distinguish them, and, in the interior of crystals, throws itself into golden-coloured radiant or circular sheaves which, when within amethyst, are the most beautiful things I know among minerals; but which it is a matter of great difficulty to distinguish in common quartz from minor forms of rutile. Finally, to crown the complexity of this iron and flint group, the sulphide of iron, varied beyond all minerals in the phantasies and grotesques which it can build out of its plastic and innumerable cubes, shoots its stellate crystals through the mass of the hydrous oxide, and disputes with it the central position in stalactites of chalcedony.

But, through all this confusion, one generalization presents itself which is of great value. Whenever iron, whether oxide or sulphide, is associated with stalactitic chalcedony, it is always in the centre of the mass; but when iron, whether oxide or sulphide, is associated with quartz crystals, it is always (if determinately placed at all), either on the outside, or at a slight depth below the surface, under an external coat of clearer crystal. It may be indeterminately placed, in dispersed stars or cubes; but, if ordered at all it is ordered so. Briefly, a crystal of quartz never has a centre of iron, and a crystal of chalcedony never a coat of it.1

And an important result seems to follow from this. If stalactites

1 Of course I do not vouch for any so wide generalization as this absolutely. If ever one ventures to do such a thing, the next stone one takes up on a dealer's counter is sure to be an exception to the announced law; but I am confident that any mineralogist can fortify the statement from his own experience quite enough to justify our reasoning upon it.

of chalcedony were formed by superfluent coats, some of these coats would have iron in solution at the outside as well as the interior, and would secrete it in successive films; whereas, on the contrary, the entire bulk of the iron, being always central, must surely have been secreted out of the entire mass; and, therefore, I believe that the true chalcedonic stalactite is indeed a long botryoidal crystal, like some of the forms of sulphide of iron, found in chalk, and not at all a drooping succession of fluent coats, except in cases of rapid deposit, which, as far as I remember, show no central iron.

Again, when iron is systematically associated with quartz, it is never in the centre of the crystal, but either on the surface or under an externally imposed glaze. Hence it follows that the crystalline forces at work in forming quartz act nearly in the reverse of those that form chalcedony, as regards the direction of ferruginous elements, and that they have quite a peculiar power in finishing crystals, which determines, at a given time, either a purer, or an amethystine, silica to the surface, often throwing down crystals of iron between the two.

I have already noticed the clear coat forming the exterior of many nested agates in basaltic cells, and the deposit of iron succeeding it, to which I gave the name of medial oxide. My impression is that the exterior of such agates, as relating to the crystalline power, may be considered identical with the centre of a stalactite, and I think it will be found that the iron in such stalactitic centres, however delicate the fibre of it, is not solid, but tubular, leaving the absolute centre of clear silica correspondent to the surface of clear silica in a quartz crystal.

It is very strange that among these complicated forces certain conditions of chalcedony and quartz should be so constant, and the intermediate states, giving evidence of formation, so rare; but though the interior of almost every quartz crystal shows the forces of agatescence and straight crystallization in confused contest, I have only seven or eight specimens, out of a collection of some thousands, which clearly show the balance of the two powers in accomplished

structure.

The uppermost figure in Plate XIX. represents a portion of one of these, which is a stellar agate, formed of grey chalcedony, with white bands collected in a knot within radiant quartz. The precision of its lines is beyond all imitation, but Mr. Allen has succeeded in drawing and engraving it for us quite well enough to show the repeated efforts of the chalcedony to throw itself into straight crystalline planes, successful, tremulously, here and there for a quarter of an inch, and then thrust again into curvature by the lateral spheric force.

The second example, engraved in the lower figure in Plate XIX., shows the two forces reconciled in their reign: the crystalline or mural form is completely taken by the agatescent bands in one part of the stone and the spheric in another, while the bands themselves are arranged in double folds, turned at the extremities, like the back of a book.

Finally, the wood-cut, Figure 4, gives the rude outline of a stone