even proved to belong to one or other of them, an analysis of the successive layers would still be wanting to demonstrate the absolute reality of the cause above explained.

Whether or not the explanation of the properties in question I have here given, be esteemed correct, there are inquiries of no little interest which spring out of the considerations above suggested. It would be important to know how far the presence of one substance in a compound body in greater or less quantity, the others remaining constant, would affect the optical properties; how far those of the compound may be affected by those of each of its constituents. In regard to form we might in the case of chabasie reason backwards, and inquire how far the whole of the silica might be rejected without change of form; whether K+ AL+6 H should be isomorphous with silica.

I have purposely abstained in the former part of this paper from adverting to the form of alumina, or to the possibility of its having a share in the production of the phænomena to be explained; but if it be isomorphous with silica, then ought K+6 H to be isomorphous with both and with chabasie. I am not aware of the existence of any hydrated oxide in a crystalline form with six atoms of water; so that we are unable as yet to say how far such an analysis of the forms of crystallized substances containing one or more constituents isomorphous with themselves, may with advantage be pursued.

Note. The above paper was communicated to the British Association at the Meeting in Dublin in August 1835*. I am now enabled to adduce two other observations which render still more probable the explanation above suggested. For the first I am indebted to Sir David Brewster, who informs me that a specimen of chabasie he has examined from Faroe, has a uniform doubly refracting structure throughout its whole mass. Now the Faroe chabasie, if that analysed by Arfvredson is to be considered a type of the whole, belongs to that kind which contains less silica, and throughout which there being supposed no excess of this latter substance above what belongs to the constitution of the mineral, no difference of doubly refracting structure of the kind above adverted to ought to be observable.

2. Among the interesting observations of Biot on the power of certain liquids to cause the plane of polarization to turn to the right or left †, is the remarkable one, that the oils of lemons

[* See Lond. and Edinb. Phil. Mag., vol. vii. p. 399.-EDIT.]

+ Translations of the Memoirs by M. Biot in which these observations are detailed, will appear in Part II. and succeeding parts of the Scientific Memoirs.-EDIT.]

Third Series. Vol. 9. No. 53. Sept. 1836.

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and turpentine produce effects represented by opposite signs, the rotation produced by oil of lemons being +, that of oil of turpentine and for a given thickness of oil only half that of oil of lemons. The effect produced by one thickness of oil of lemons is almost exactly neutralized by two thicknesses or twice the weight of oil of turpentine: and this whether the two liquids be in separate tubes, or be previously mixed and presented to the ray of light in one and the same tube. This is precisely analogous to what, in the above paper, we suppose to take place between the molecules of quartz and those of chabasie. Portobello, July 22, 1836.

XXXVI. Observations relative to the preceding Paper. By Sir DAVID BREWSTER.

IN examining the optical properties of the different chabasies, I found that the variety from the Giant's Causeway differed so essentially from the ordinary kinds as to entitle it to the distinction of a new mineral. Its double refraction was considerably greater, and its ordinary refraction considerably less than that of the common kind. I have since had occasion to examine a most interesting variety of chabasie brought from Faroe, and presented to me by W. C. Trevelyan, Esq. It consists of minute and perfect rhombs sticking loosely together, as it were in stalactites, and these minute crystals are perfectly transparent and much better crystallized than any other specimens which I have seen. Each rhomb, however, was a composite crystal, and the faces of composition coincided with the diagonals of its rhomboidal faces.

The remarkable property which Mr. Johnston has referred to in the preceding ingenious paper, does not exist in these minute crystals from Faroe, and I did not observe it in the specimens from the Giant's Causeway. If it exists, therefore, only in the common chabasie, it will not be difficult to put Mr. Johnston's hypothesis to the test of direct experiment, because this chabasie must, if the hypothesis be true, contain a greater quantity of silex than the other varieties; in order, however, to establish the hypothesis it must also be proved that the outer layers of the rhomb contain more silex than the inner rhomb or nucleus; and if the additional quantity of silex is very small, it may exist as an extraneous ingredient, diminishing its double refraction, not by an opposite double refraction of its own, but merely by separating the particles of chabasie, and diminishing the force of aggregation on which the double refraction of the mineral has been supposed to depend*. Edinburgh, July 23, 1836.

See Phil. Trans. 1830, p. 93. [An abstract of the paper here referred towill be found in Phil. Mag, and Annals, N.S. vol. vii, p. 356.-EDIT]

XXXVII. Reply to Dr. Boase's "Remarks on Mr. Hopkins's 'Researches in Physical Geology," in the Number for July. By W. HOPKINS, Esq., M.A., F.G.S., of St. Peter's College, Cambridge.

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To the Editors of the Philosophical Magazine and Journal.

GENTLEMEN,

BEG to reply through the medium of your Journal to some remarks by Dr. Boase, contained in your last Number [for July], p. 4, on my memoir on Physical Geology, and to add some additional facts which I have recently had an opportunity of observing.

Dr. Boase appears in the first place to object to the hypo thesis on which the whole of my investigations are founded, that of the simultaneous action of an elevatory force on the exterior crust of the globe throughout regions of considerable extent, because he conceives that extensive dislocations produced by such a force would probably be attended with enormous convulsions proportioned to the extent of the rupture. This objection rests entirely on an assumption as to the intensity of the elevatory force. Dr. Boase has not given any reason for supposing that explosion must probably accompany dislocation, and without some such reason it is certain that we have no right to make the assumption. It is obvious, in fact, that we can have no means whatever of judging of the intensity of the elevatory force except by the effect produced by it. For anything we can know of its nature independently of inference from observed phænomena, it might be insufficient to produce an earthquake or adequate to produce an almost universal volcano. It may be observed, however, that the extent of simultaneous dislocation would do more than anything else to counteract the explosive tendency of an expansive fluid, because the more extensive the dislocations the more rapidly would the force of expansion be diminished, and the more equable would be the effect on the whole mass. If, on the contrary, a small portion only of the mass should give way, the expansive force would be but little diminished, and its continued action on the yielding part would unquestionably produce much more violent effects on that part than if the mass had yielded generally.

Dr. Boase has rested his objection partly also on the notion, that according to my views the fissures must necessarily begin at the under or lowest side of the elevated mass. He will find it carefully stated, however, in my memoir, that they "will

not commence at the surface but at some lower part of the mass. If the extensibility of the lower part of it be sufficiently increased by its higher temperature, the fissures will commence at some point between the upper and lower surfaces, and will be propagated both upwards and downwards, and may or may not, according to the degree of extension of the mass, reach either the upper or lower surface. This is a point of little consequence as far as regards Dr. Boase's objection above stated, but it may serve to account for the fact that eruptions of fluid matter in some cases have, and in others have not, accompanied dislocations and elevatory movements.

In my investigations I have spoken of elevatory forces, the idea of the higher portions of the earth's crust having been elevated being in general, perhaps, more familiar to us than that of the lower having been depressed. I would here observe, however, that so far as relates to the first production of fissures it is immaterial whether we suppose the mass to be bent upwards by a force beneath, or downwards by its own weight, provided the regions thus subsiding simultaneously be of the same extent as those which I have always spoken of as being simultaneously elevated. The secondary phænomena, however, resulting from the fissures produced by the upward, and by the downward movements respectively, would probably present certain characteristic differences; but I shall not now enter into any further investigation of them.

In the abstract (Art. V.) of my memoir which appeared in your Journal (vol. viii. p. 359), I have taken considerable pains to indicate the possible influence of a jointed structure existing in the elevated mass previously to its elevation, and how it might be ascertained by observation, whether or not this influence had been considerable. Dr. Boase, however, is disposed to arrive at the determination of this point by the shorter, but, in such matters, most unsatisfactory road of à priori reasoning. He observes: "If then solid rocks have necessarily a jointed structure, one of the data on which Mr. Hopkins's calculations are founded is invalidated, in as much as the elevating force can never have acted on a solid mass without the interference of this modifying circumstance." Now, in the first place I would observe that the process of solidification of all rocks must necessarily have been extremely slow, and that probably, therefore, all the modifications which they have undergone during that process must have been the gradual work of lengthened periods of time. It is impossible, then, to say what period might be necessary for any portion of the earth's crust to arrive at that state of its jointed structure which should produce any decided effect on the directions of its dis

locations. In my investigations it is unnecessary to suppose any but the lowest degree of solidification in the elevated mass; and therefore it is manifestly quite inadmissible to assume that it could not be dislocated by an elevatory force before its jointed structure had become sufficiently developed to determine the directions of dislocation. Yet the only force which can possibly attach to the objection above quoted depends entirely on this assumption, which, in fact, involves the very point at issue, viz. whether the jointed structure of disturbed masses has been in great measure superinduced previously or subsequently to their elevation. It is not, however, by this kind of à priori reasoning, founded on what we are altogether ignorant of, that the merits of geological theories can be determined; and to attempt to do so is to depart from those principles of inductive philosophy which alone have enabled man to comprehend with clearness and precision so much that is beautiful and wonderful in the laws of nature. I have elsewhere stated that I have not entered into these discussions in the spirit of advocacy of preconceived opinions; and with respect to the two theories, of which one would assign the directions of dislocation principally to the manner in which the elevating force has acted, and the other to the previously jointed state of the mass, I have endeavoured to act with perfect impartiality. I have indicated how their relative claims may probably be decided by observation, by which alone, I assert, these claims can be determined, and not by the kind of reasoning on which both the objections above noticed are founded.

With a view to this determination I have lately made some careful observations in the limestone and gritstone district of Derbyshire. In a particular and thick mass of limestone which pervades the greater part of that inining district, the joints are remarkably well developed. They form two systems at right angles (or very nearly so) to each other, which preserve their directions with remarkable accuracy in every part of the district. The other beds also have their principal joints in the same directions wherever they can be distinctly recognised; and such also is the case with the immense mass of gritstone superincumbent on the shale and limestone. One of these directions is a little west of the magnetic north; the other being consequently a little north of magnetic east, while the directions of all the characteristic dislocations of the district are nearly east and west and north and south, thus deviating from those of the joints by an angle of from 20° to 30°, precisely of that magnitude which is too large to be possibly attributed to any error of observation, and too small to admit