several definite rays, among which two green rays appeared to me to possess nearly the same tint, although differing in refrangibility.

Gold-leaf, and copper-leaf each afforded a fine spectrum exhibiting peculiar definite rays. The effect of zinc was still more interesting; I observed in this instance a strong red ray, three blue rays, besides several more of other colours. These experiments were made in the laboratory of the Royal Institution in June 1834.

Errata. In the memoir on Light, vol. v. p. 326, line 22, for "jointed" read "joined."

P. 328, line 3 from the bottom, for "observations" read "obscurations".

The conclusion of the article in vol. iv. p. 290, is inaccurately expressed; it should stand thus: "But when the obliquity reaches a certain point one of the images suffers total internal reflexion before the other does. This would equally happen whether the balsam were employed or not: but its use is attended with advantage because this effect then takes place at a much greater angle of obliquity, and consequently the separation of the images is more perfectly and conveniently obtained."

II. Remarks on Mr. Hopkins's "Researches in Physical Geology." By HENRY S. BOASE, M.D., &c., Secretary to the Royal Geological Society of Cornwall.*

HE vast accumulation of facts during the present century concerning the structure of the earth, has established the title of geology to rank among the more important branches of human knowledge: and the attempts which have been lately made to solve geological problems by mathematical analysis, lead us to expect that it may one day attain to a more exalted position among the sciences. In Germany, this application was made many years ago, more particularly by Schmidt and Zimmermann, in illustration of the phænomena of veins. And in France, Elie de Beaumont and Dufrenoy have called in its aid to support the doctrine of elevation-craters. But to Mr. Hopkins we are greatly indebted for vindicating the character of the science in this country by making a more extended application of this agent, and for the important results at which he has arrived.

* Communicated by the Author.

+ Mr. Hopkins's Abstract of his Memoir on Physical Geology appeared in our last volume, p. 227, et seq.

I am not duly qualified to offer an opinion concerning the mathematical portion of Mr. Hopkins's work: but, admitting the great merit of the application of his propositions in pointing out what state of things is, or is not, compatible with the action of an elevatory force, I would venture to offer some remarks respecting the accuracy of the data on which his reasoning is founded. And I the more readily enter on this examination, as he has stated that his "speculations are thrown out with the hope of indicating some of the more critical points of inquiry on which the ultimate determination of this question must turn, and which are generally best indicated in such cases by theoretical discussion."

I have, in my " Treatise on Primary Geology," expressed an opinion that many supposed cases of elevation may be explained by the lines of structure, and by depositions on uneven surfaces; but waiving this at present, I will at once proceed to consider the nature of the elevatory force and its modes of action, adopted by Mr. Hopkins as the basis of his investigations.

The elevatory force which acted upon the lower surface of the uplifted mass is supposed to have been some expansive fluid. Now until the mass be ruptured this may be a satisfactory cause; but is it not probable that immediately the resistance has been overcome by the production of fissures, an explosion would follow, which would be as much more terrific in its devastations than modern volcanos, as the vent, extending across whole continents, would vastly exceed the dimensions of a crater? Can the present state of things-strata dipping on either side of mountain ranges traversed and intersected by regular systems of veins and dykes-be supposed to have resulted from such tremendous convulsions? And again, the internal pressure being removed by the rupture of the elevated mass, and the support consequently withdrawn, ought not the strata to have fallen down and have obliterated the spaces in which the veins and ranges of granite are said to have been subsequently formed?

This objection might be, in some measure, got over by supposing the uplifted and fissured mass to have been acted on whilst a solid layer was interposed between it and the moving power, whereby the fractured strata would be supported; but such an expedient, even if admissible, would only be an imaginary solution of the difficulty, and would be at variance with Mr. Hopkins's deduction that the fissures must necessarily commence in the under part of the mass.

Granting, however, the probability of this modus operandi, and the accuracy of the first approximation, we will proceed to

nomena.

the modifying circumstances, the possible existence of which Mr. Hopkins has admitted, and stated that they would render it very difficult to calculate with any precision the resulting phæ→ One of the most influential of these circumstances is the jointed structure in the mass subjected to the action of the elevatory power. Mr. Hopkins, duly impressed with its importance, has made many interesting remarks thereon, more particularly concerning the coincidence of lines of structure and of fissure.

The structure of rocks is a subject which has engaged my attention for many years; and in the seventh volume of this Journal (p. 376 et seq.) I endeavoured to show that the joints have not been mechanically produced, as Prof. Sedgwick supposes, but have resulted from an arrangement of the particles by the attraction of cohesion during the process of consolidation, being in its nature similar to crystallization. The Professor has, indeed, referred to Mr. Hopkins's "Researches" in support of his views, but it will be seen by the following passage that Mr. Hopkins, on physical grounds, has arrived at the same conclusion as myself. "It has been shown that extraneous forces could only tend to produce systems of fissures crossing each other at right angles, whereas regular systems of joints appear to meet each other frequently at acute angles, and consequently must necessarily have been owing to some different cause. I do not therefore conceive that any general tension of the mass produced by extension from elevation, or contraction in the course of solidification, can have had any material effect on the formation of joints. It is probably, I think, to be referred entirely to some kind of internal molecular action."

That the lines of structure are perfectly independent of any elevatory power would seem to be implied by their occurring even inclined in opposite directions entirely within horizontal strata; and also by their existence in solid rocks between beds of incoherent substances, as, for instance, in the oolite of Buckland Point, in the parish of Mells, which Mr. Townsend has described as rhombohedral beds dipping at an angle of 40°, and confined between two horizontal beds of clay*.

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 elevatory force can never have acted on a solid mass without the interference of this modifying circumstance. The abstract consideration of the question I can easily conceive to be required in order to ar

Greenough's Geological Essays, p. 13.

rive at a just estimation of the individual value of the moving power; but it would seem to be almost impracticable to calculate the result of such an action on a mass traversed by joints at acute angles to each other,-a condition found to be very prevalent in existing rocks. I am aware that it is not generally admitted that all solid rocks have a jointed structure; but Mr. Hopkins ought not to object to it, as he regards such structure to have proceeded from an "internal molecular ac

tion."

Now, if rocks having a jointed structure be acted on by an expansive fluid so as to overcome their continuity, it might be expected, as I have elsewhere stated, that the dislocations would take place on the lines of structure, as being the directions of least resistance. Mr. Hopkins also adds an excellent practical remark, " that the accuracy of coincidence, between the lines of structure and of fissure, is essential to the theory which would assign the latter phænomena to the prior existence of the former. A difference of a few degrees in the angular position of the above lines would, if clearly established, be fatal to this theory: because, as I have already explained, although a fissure produced by an elevatory force would cross a line of less resistance under a certain condition without change of direction, that condition cannot be generally satisfied when the angle between the fissure and line of less resistance is small; in such a case the fissure will be propagated exactly along the latter line." Such an occurrence, it appears to me, might rather suggest a doubt concerning the supposed action of an elevatory force, since our daily experience shows that all solid rocks do now possess a jointed structure. Mr. Hopkins, however, is of opinion that should this coincidence be established, it is more probable that the lines of structure have been influenced by the dislocation of the mass than vice versa. This result of the mathematical analysis seems to have reduced the argument in favour of an elevatory force ad absurdum; for undisturbed horizontal strata have a jointed structure, and in rocks having a rhomboidal structure the joints meet at acute angles, which Mr. Hopkins himself states must necessarily have been owing to some different cause than that of extraneous forces, such as extension from elevation or contraction during solidification.

Mr. Hopkins has visited Cornwall, and found it, like others, a stumbling-block. He does not view its perplexing phanomena as indicative of a defect in the theory of geology, but regards them as exceptions to the general rule. "In this mining district," he says, "it appears most necessary to re

cognise the influence of a previously veined or jointed structure on the direction of its dislocations." And yet two or three pages afterwards we find a different opinion, viz. that the great system of metalliferous veins was formed in open joints superinduced after the great dislocations which accompanied the injection of the granite. I am glad to find that his "hasty inspection" of Cornwall has confirmed my more lengthened observation on the coincidence of the directions of veins with those of joints; but he surely jumps too hastily to the conclusion that this circumstance destroys the contemporaneous hypothesis which I have advocated. Although the direction of veins may correspond with lines of structure, they are not identical therewith. The joints pass indiscriminately through both veins and rock, whether granite or slate, or both conjointly; so that individual concretions or blocks, formed by the intersection of parallel systems of joints, contain more or less of these, according as the vein contracts or expands in its dimensions:-just as the lines of structure in sedimentary rocks pass through concretions and organic remains, so do they traverse the granite, slate, and veins, showing in both cases that the things intersected must have existed previous to the consolidation of the mass.

Mr. Hopkins regards the contemporaneous formation as "an inconceivable process," more especially for those who consider the slate as a sedimentary rock. Such a notion, however, is not perhaps so absurd as it at first sight appears to be, but is consistent with the prevailing theory; for if the primary slates be metamorphic rocks, why not the granite also? Vast districts in the North of Europe and elsewhere consist of gneiss, which some have called granite, and others granitic gneiss. Why was it not as easy for the central fire to change fossiliferous strata into granite, as into granitic gneiss? And if so, then the primary slates and granite might be contemporaneous; though in this case formed by a superinduced or secondary action, whilst in the one which I advocate they have resulted from the original fusion of the earth. Mr. Hopkins acknowledges that "the perfect continuity of the veins of Cornwall in passing from the killas to the granite forms a curious feature in the geology of that district if we are to regard the former as a sedimentary deposit." It certainly places the prevailing theory in a most perplexing dilemma, which however it appears to me is satisfactorily solved by admitting the contemporaneous nature of the granite and slate.

As the metalliferous veins of Cornwall do not seem to be