Report on the Geological Theories of Elevation and Earthquakes. Part I.-On the Phænomena and Theories of Volcanos; and on the Form, Section I. On the Phænomena and Theories of Volcanos. ANY theoretical exposition of the geological phænomena of elevation, and the physical causes to which they may be attributable, would seem to be imperfect without some attempt to explain the origin of the forces which have elevated and fractured the crust of the globe, as well as the mechanical effects resulting from them. I shall therefore avail myself of the range which I understand the Association to allow me in making this Report, to consider, in the first place, the probable nature and origin of the mechanical agency in question. And here I may remark, that in speculating on the causes which have operated at past geological epochs, we are naturally led to refer to present causes as at least suggestive of the probable nature of those which have formerly acted, however remote may have been the period of their action. Thus in speculating on the physical causes which have produced those dislocations which so curiously characterize the surface of the earth, our attention is directed to existing volcanos as likely to indicate the agency by which the phænomena in question have been produced. The frequent existence, in highly disturbed districts, of masses of rock which have been apparently injected into the general mass of sedimentary deposits, or ejected over their surface, and which are now universally allowed by geologists to be of igneous origin, presents analogies with the effects of volcanic action at the present time which cannot fail to suggest the idea that the general phænomena of elevation are attributable to the same kind of action. I shall therefore enter, in the first instance, into such considerations on the characters of volcanos and their modes of action as may appear to bear immediately on the subject of the phænomena of elevation generally. 1. Volcanos.-A volcano consists of a mass of fluid lava contained in a cavity of the solid crust of the earth. The most important characters which we are here called upon to recognise in volcanos are the following:— (1.) The cavities containing the fluid matter have direct communication with the external air by means of volcanic vents. The dimensions of the cavities are unknown. (2.) The volcanic mass is maintained in a state of fluidity by its elevated temperature; and its composition is such that the formation of elastic gases is constantly taking place, if not in every part of the mass, at least to a considerable depth beneath its surface. (3.) The elastic force which these gases are capable of exerting at the high temperature of the mass in which they are generated is extremely great. 2. With respect to the containing cavity, we may observe that its vent is an essential character, that on which the mode of action more especially termed volcanic depends. Respecting the. dimensions of the cavity, very different opinions have been maintained. Some have considered the fluid matter of all volcanos to communicate with, or rather to be a part of, a central fluid nucleus constituting the great mass of the earth, and enveloped by a solid shell of not more than a few leagues in thickness; while others have supposed, independently of theoretical views respecting the existence of a fluid nucleus, that a communication exists between volcanos not too far re1847. D moved from each other, much more generally than superficial appearances may seem to indicate. The hypothesis that the thickness of the solid crust is extremely small would seem essential to give any degree of probability to the first of these views, but I shall shortly offer reasons for believing such an hypothesis to be untenable; and independently of this consideration, it is manifest that if that view of the subject were correct, the upper surface of the fluid mass of every volcano not in a state of eruption must be a portion of the same surface of equal pressure, and consequently at the same height above the level of the sea; nor would it seem to be possible for an eruption to take place at a great elevation in one volcano, without producing simultaneous eruptions at least from neighbouring volcanos situated at lower levels. The same remarks would be applicable to any local group of volcanos freely communicating with each other, except that the common level of the surfaces of the volcanic fluid in one such group might be different from that of another, the communication being independent of a common fluid nucleus. It does not appear, however, that we have the slightest evidence of an equality of altitude of the incandescent mass of neighbouring volcanos, and it is certain that there is no synchronism in the epochs of their eruptions. The great eruptions of Etna have not been accompanied by simultaneous eruptions of Vesuvius, while the intermediate volcano of Stromboli has preserved its comparative tranquillity during the most violent disturbances of its neighbours, and remained without a single recorded eruption during 2000 years; and possibly this comparative repose may have been of much longer duration. Such facts as these would seem to prove beyond doubt that the fluid nuclei of different volcanos have generally no perfectly free communication with each other, and may so far, in their actual state, be regarded as insulated phænomena. In what degree this insulation may have originally existed, or how far it may have been the result of physical causes acting during long periods of time, are questions which we shall have hereafter to discuss. 3. With reference to the gases above mentioned, we may remark that we are not here concerned in any degree with their chemical nature, but merely with their property of elasticity. Distinct evidence of the continuous generation of gases in the interior of the volcanic mass, and their consequent continuous ascent to the surface, is afforded by such volcanos as that of Stromboli and Kirauea*, which remain for long periods of time in sensibly the same state, uninterrupted by those periodical explosive eruptions which form the most striking feature in volcanic action. The fluid masses in these volcanos remain at nearly a constant elevation, and are observed to be in a constant state of comparatively quiet ebullition, arising, undoubtedly, from the continuous disengagement of elastic vapours formed within them. The fact also of the constant escape of such vapours from volcanic foci is well established by observation. 4. That the fluidity of the volcanic mass is due to heat is universally allowed, but it has been doubted, and with apparent reason, whether it be precisely of the same nature as the fluidity of a perfect fluid, or that of an ordi * A volcano in Owhyhee, one of the Sandwich Islands. An interesting account of it is given by Captain Tarleton in Blackwood's Magazine (November 1845). It is also described in Ellis's Narrative of a Tour in the Sandwich Islands, p. 199. In this volcano, as well as in that of Stromboli, the surface of the fluid lava is visible. They consequently afford important evidence respecting the position of the surface of the volcanic fluid with reference to the level of the ocean. Volcanos in general afford no evidence of this kind during the periods of comparative repose which intervene between their eruptions. Kirauea is 4000 feet above the level of the sea. nary metal in a state of perfect fusion. In the latter case it is conceived that no particle of finite dimensions retains its solidity, whereas in the case of fluid lava it has been supposed that a large portion of the mass consists of small but finite particles, which retain severally their solidity, while their relative mobility is maintained by the remaining portion of the mass intervening in a more perfect state of fluidity between the solid particles, and consisting partly of internal elastic vapours, which by their ascending movement keep the component particles of the mass in a constant state of ebullition, and give to it an apparent fluidity, much greater than that of lava, immediately after its ejection from a crater*. Be this however as it may, it is important to remark the influence which this constant movement of internal gases must exercise in counteracting the tendency of the upper portion of the mass to solidify in consequence of the loss of heat. It should also be remarked how much this influence may be increased by the contraction of the containing cavity as it rises towards the external vent. For the currents of ascending vapours, in their ascent to the surface, will produce a degree of internal movement inversely proportional to the area of the horizontal sections through which they pass; and therefore, whenever the fluid mass rises into the narrower part of the volcanic vent, these currents will preserve the more superficial part of the fluid in a state of ebullition the more violent in proportion to the contraction of the channel through which the vapours ultimately escape. Thus the incipient solidification or freezing of the surface of the fluid may be prevented long after the temperature has descended to that which would be the temperature of fusion of the mass if it were placed in a state of perfect rest. 5. The enormous expansive force which the elastic vapours of volcanos are capable of exerting is attested by the violence of volcanic eruptions. When disengaged from the fluid lava their power is shown by the velocity with which solid masses of rock, as well as looser materials, are frequently projected from the mouths of volcanos; and while existing within the fluid mass, their hydrostatic power is distinctly proved by the column of lava which they must have to support during the time of its overflowing from the mouth of an elevated volcano. When we further consider how rapidly the expansive force of vapour increases with its temperature, we may conceive how enormous a force these volcanic gases may have been capable of producing at former geological epochs, if we allow that the temperature of the earth's crust may then have been considerably greater than at the present time. 6. Mode of Action of Volcanic Forces.-The immediate effects of the more energetic action of volcanic forces are so generally exhibited in the form of violent explosions, that we might, without due consideration of the subject, be led to suppose that sudden and explosive action constituted an essential character of such forces. It must be recollected, however, that to produce an explosion two conditions are necessary. In the first place, a force of great intensity must be produced almost instantaneously, or the resistance opposing a force already in action as instantaneously removed; and secondly, the mass on which the force acts must be so small as to admit of a very great velocity being almost instantaneously communicated to it. A force like that in question is suddenly produced by the ignition of an explosive compound, and the resistance to an enormous expansive force is as suddenly removed by the bursting of a boiler filled with steam at a high tension; but in neither case would an explosion be the result, unless the mass acted on were small. We * See Scrope' On Volcanos,' p. 19. must not therefore conclude, that because volcanic forces produce violent explosions when acting under the actual conditions which existing volcanos present, they would produce similar effects under different conditions. Such would certainly not be the case if the inertia of the masses on which the forces acted should be sufficiently great*. The permanent state of ebullition and gentle intumescence observed in Stromboli, Kirauea, and a few other volcanos, is manifestly the consequence of the constant generation of elastic vapours, and their regular ascent to the surface through which they escape into the atmosphere; but it is difficult to obtain any evidence respecting the precise cause of that discontinuous and paroxysmal action which constitutes the usual characteristic of volcanic action. Perhaps a careful study of the great intermittent hot springs of Iceland, the Geysers, is as likely as any means we possess, to elucidate the subject. One explanation which has been given of the intermittent action of these springs is founded on the hypothesis of the existence of reservoirs of steam in the internal cavities, with which the external vent, in the form of a long cylindrical tube, communicates. These reservoirs may be conceived to be formed, in some cases, in those parts of internal cavities where the roof rises higher than the outlet. Steam may be formed in them after each explosion, of sufficient elastic power (if the heat be great enough) to press down the surface of the water at first filling the cavity, till it descend to the level of the outlet, through which a portion of the steam may then make its escape. The steam thus escaping will rise rapidly through the water, to escape by the external vent. Under these circumstances, if the quantity of steam which escapes at the same instant, and its elastic power, be sufficiently great, and the water rise into the narrow cylindrical tube, the water occupying the tube will be expelled by the steam in exactly the same manner as the contents of a gun-barrel are expelled by the elastic force of the vapour instantaneously generated by the ignition of the gunpowder employed. The conditions assumed respecting the form of the internal cavities appear highly probable, and the accumulation of steam in them would probably be a necessary consequence of their existence. The part of the explanation which appears to me least satisfactory, is that which assumes the sudden escape of the accumulated vapour in sufficient quantity to produce the observed effects, and so far to exhaust these reservoirs as to render necessary considerable periods of repose before the renewal of a similar action. The above explanation is equally applicable to the intermittent action of volcanos; and perhaps the difficulty just mentioned may not be so great as in springs like the Geysers, because the tension of the elastic vapours in the former must be enormously greater than in the latter case, and may possibly in the deep regions of a volcano produce disruptions in the containing rocks by which the vapour may be suddenly liberated in sufficient quantity to produce a violent eruption. Possibly however the explosive action of volcanos may be connected with some modification yet unknown to us in the process of generating elastic vapour when that process takes place under high pressure t. 7. Theories of Volcanos.-Fundamental Hypotheses.-It is the object of physical science in general to explain the phænomena which the physical * In the removal of the chalk cliff near Dover for the purpose of constructing the Dover railway, there was, I believe, scarcely any appearance whatever of what would commonly be regarded as an explosive action of the gunpowder employed for the purpose. In the Annales de Chimie for last April, MM. Descloiseaux and Bunsen have given a different explanation of the intermittent action of the Geysers. I content myself with this reference to it, because however admissible it may appear with respect to those springs, it will probably be regarded as much less so as an explanation of volcanic eruptions. world presents to us, by referring them to the agency of natural causes; but in our more extended speculations on geological causation we are necessarily driven to the consideration of the limits beyond which we are compelled to recognise the operation of those higher causes of which physical science can take no cognizance. It is far from my object to enter into a discussion of questions of this nature, but before we can proceed to the examination of theoretical views, and the hypotheses on which they are founded, it is essential that we should have some rule for our guidance on this point. The principle I would lay down is this,-That in treating on the natural phænomena which geology presents to us as objects of physical investigation, we must refer them to natural causes, so far as it can be proved that the phænomena would be the necessary consequences of such causes acting under conditions the former existence of which may be deemed admissible. But in the application of this principle we are bound to be especially careful that the dependence of the phænomena observed on the causes assigned be established by accurate reasoning, and not by vague assumptions. It is to the gross neglect of what the rule as above enunciated dictates in this respect, that we must attribute so many indeterminate speculations and objectionable opinions which have been offered to the world as geological theories. It is easy to substitute speculative notions for rigorous demonstration, but let us not forget that no one can thus build up geological theories without violating all the rules of inductive philosophy, and overstepping the caution and modesty which are essential for our safe guidance in these remote regions of physical science. There are two fundamental hypotheses on which theories of volcanos have been founded one of them assumes the earth at some former epoch to have been in a state of fluidity; the other assumes it to have been originally constituted with an unoxidized solid nucleus. The first of these hypotheses, it will be observed, does not assume a fluid state to have been the primitive state of the matter composing our planet, but merely one antecedent to its present state, and manifestly does not interfere with the hypothesis of a still anterior gaseous state of the earth, or of the matter composing the whole solar system. If the second hypothesis be intended also not to define the absolutely primitive state of the globe, it may be demanded of the theory founded on it, that it should give at least some probable explanation of the assumed existence of the solid unoxidized nucleus as the consequence of a previous state of the terrestrial mass. If, on the contrary, the state assumed by this hypothesis be regarded as the primitive state of the earth, the hypothesis would seem in itself to be less simple than that which, without professing to define the primitive state of matter, supposes our planet to have once been gaseous or fluid. According to this latter hypothesis, the terrestrial mass must have been more entirely free to receive all the modifications of constitution and form which physical and mechanical causes may have tended to impress upon it up to the present epoch, than if it had always existed in a state of solidity; and consequently this hypothesis (considered merely as a fundamental assumption) must I conceive be preferred to any more restrictive one by those who are disposed to adopt the principle enunciated in the preceding paragraph, respecting the recognition of secondary causes as the immediate agency employed in the production of natural phænomena, whenever it can be proved that the phænomena would result from such causes acting under admissible conditions. It must not be supposed that I would determine the value of an hypothesis. by antecedent considerations, independently of the inductive proofs by which it may be supported; on the contrary, I would insist on such proofs as the |