to have been filled from below, or the lower part filled from above? Either the one hypothesis or the other appears totally inadmissible, unless we suppose the communication between the upper and lower parts of the vein to have been formerly very much more perfect than at present. This hypothesis would, perhaps, present no very serious difficulty, because it is very possible to conceive the toadstone to have been so imperfectly solidified at the time of the formation of these fissures, as afterwards to diminish their width, by yielding in some measure under the pressure of the superincumbent mass. But if we suppose the portions of the fissure both above and below the toadstone to have been filled either from above or below, while there existed a wider fissure connecting them through the toadstone, this fissure in the toadstone must also have been filled before its ultimate degree of contraction, in which case it appears almost impossible that there should not be a much more determinate trace of a vein through the toadstone, than is at present observed to exist. We seem almost necessarily driven in these cases to the hypothesis of some process of segregation or infiltration into fissures previously formed for the reception of the segregated or infiltrated matter.

§. On the Formation of Granite Veins.

73. These veins have been described (Introd. VII.) as distinguished in general by the absence of that tendency to rectilinearity and parallelism in their directions which so distinctly characterize the principal mineral veins in each mining district. The fact of these veins being found only at the junction of masses of granite with other masses of different mineralogical constitution, has naturally suggested the idea of these veins being veins of injection; the granite being assumed to be of igneous origin. This opinion seems strictly in accordance with the views which we have been developing. The rectilinearity of mineral veins is due, according to this theory, to the predominance of tensions acting in a particular direction, whereas fissures formed in great measure by the hydrostatic pressure of injected fluid matter, in a mass subjected to no tension very determinate in its direction, might assume

any tortuous course. The irregular and violent action, also, to which the mass through which, according to this view of the subject, the granite is supposed to have been protruded, would have a great tendency, independently of the hydrostatic pressure just mentioned, to form in the broken mass irregular fissures, which would facilitate the injection of the fluid matter, and increase the irregularity of the form of the injected veins.

§. On the Formation of Trap-Dykes and Veins.

74. The results above obtained respecting the formation of fissures in the crust of the globe will manifestly hold equally, whether we suppose the uplifted mass acted upon immediately through the medium of an elastic vapour, or by matter in a state of fusion in immediate contact with its lower surface. In the latter case, however, this fused matter will necessarily ascend into the fissures, and if maintained there till it cools and solidifies, will present such phenomena as we now recognize in dykes and veins of trap. The same phenomena would result from the injection of the fluid matter at any period posterior to that of the formation of the fissures as above described. To represent to ourselves, therefore, the phenomena of trap-veins, as referred to the causes to which we are referring them, we have only to conceive the fissures previously described filled with trap. The larger ones will thus form dykes, and the smaller ones veins of that rock.

75. It has been observed by geologists, and particularly by M'Culloch, that a large proportion of trap-dykes have been formed without producing any sensible disturbance in the ends of the stratified masses abutting against them. And this is precisely what we might expect, if we suppose such dykes to have been injected without excessive violence into fissures formed as above described, whether that injection be supposed to have taken place after the formation of the fissures, or contemporaneously with it. Where injection, however, has taken place in great abundance, and with great violence, corresponding degrees of disturbance might of course be expected to attend it.

The geologist to whom I have just referred, in speaking of the trap-veins of the Isle of Sky, observes: "It is necessary to point out one extraordinary effect which must have resulted from the intrusion of these veins. Whatever proportion, collectively taken, they may bear in breadth to the lateral dimension of the strata which they intersect, it is plain that the whole mass of strata must have undergone a lateral extension equal to that quantity; a motion so great as not to be easily reconciled with the present regularity of the whole. It is also a singular circumstance, that on the opposed shore of Sleat a different effect takes place, and proportioned, it would here seem, to the number of veins; the red-sandstone strata of this coast being often turned from a slightly inclined into a nearly vertical direction, with other considerable marks of disturbance. It is impossible to account for these apparently capricious differences, and we must for the present be content to rank them among the numerous unexplained phenomena in which the science abounds."

These phenomena present no difficulty except in the apparent lateral displacement of the stratified beds, without any other appearance of disturbance; and if this effect is to be referred to the lateral pressure of the injected matter, it does indeed present a difficulty no less, I conceive, than a physical impossibility. In the first place, it appears inconceivable how sufficient resistance could be obtained from above to produce the enormous lateral fluid pressure necessary to cause this lateral movement, as we have before remarked respecting the horizontal heaves of mineral veins; and in the next place, it is still more inconceivable how this force could have been exerted without indications of such violent action. Under the point of view, however, in which I have regarded the subject the difficulty no longer exists; for it must be recollected that the aggregate width of the veins, or apparent lateral displacement, is not to be taken with reference to the breadth of the mass in which the veins immediately exist, but with reference to the whole extent of the mass, the tension of which may have been relieved by the formation of these fissures. No rational account can be given, I conceive, of such lateral movements of extensive masses, except by referring them to the horizontal tension produced by vertical forces, and

the consequent contraction when the mass becomes fractured by too great an extension.

§. On the Formation of Horizontal Beds of Trap-By Ejection-By Injection-Remarks on some Phenomena observed by M'CullochEffect of imperfect Fluidity in Horizontal Injections.

76. If the quantity of fluid matter forced into these fissures be more than they can contain, it will of course be ejected over the surface; and if this ejection take place from a considerable number of fissures, and over a tolerably even surface, it is easy to conceive the formation of a bed of the ejected matter of moderate and tolerably uniform thickness, and of any extent. If the ejection take place over a level surface, these properties of the resulting bed would seem to require a number of points or lines of ejection as a necessary condition, on account of the imperfect fluidity, which, according to analogy, we ought probably to assign to the ejected matter. If there were only a single center of eruption, a bed of such matter approximating to uniformity of thickness, could only be produced on a surface of a conical form, having the point of eruption at its vertex, and an angular elevation depending on the degree in which the fluidity of the ejected mass should differ from perfect fluidity. Where no such tendency to this conical structure can be traced, it would probably be in vain to look for any single center of ejection. On the supposition too, of ejection through continued fissures, or from a number of points, that minor unevenness of surface which must probably have existed under all circumstances during the formation of the earth's crust, would not necessarily destroy the continuity of a comparatively thin extensive bed of the ejected matter, in the same degree in which it would inevitably produce that effect in the case of central ejection.

77. I will now proceed to consider the formation of a horizontal bed by injection; what limits may be imposed on the probable or possible extent of it, and with what phenomena it may be accompanied, which may serve as tests for distinguishing a bed so formed from one formed by ejection over the external surface.

Let us suppose then, that the fluid mass has risen through the fissure of which Cc is the section, till it has reached the stratum adb. If this stratum have sufficient tenacity and extensibility, and but little adhesion to that on which it reposes, it is easy to conceive that it may be elevated without being broken, if the fluid mass be impelled upwards with sufficient force to overcome the weight of the superincumbent mass. In this case the fluid will necessarily be injected horizontally, as represented in the figure, and so long as the lower surface of the uplifted stratum remains perfectly continuous and unbroken, it is very possible that this injection may extend to any assignable distance without the

production of vertical dykes, on veins branching from the upper surface of the injected bed. In this case there would appear to be no indications of mechanical action from which the geologist of the present day could ascertain whether such bed had been injected among the beds associated with it, or ejected over the surface acb at a period anterior to the formation of the superincumbent strata.

The most favorable case we can conceive for the kind of injection we are considering, without the production of the vertical veins above mentioned, is that in which we assume the absence of all adhesion between the uplifted bed and that immediately beneath it; but even in this case the condition of unbroken continuity in the lower surface of the superincumbent mass, must be satisfied, not approximately, but accurately; for if the smallest crevice existed in the uplifted portion, the injected matter would be impelled into it with a force proportional to the enormous pressure to which it would be subjected from the weight of the superincumbent beds; and if the injection should take place under the weight also of a deep sea, the probability of this effect