the earth is not less than from a thousand to twenty-five hundred miles thick is open to question. We can not imagine that a crust of that enormous thickness could, in such recent geological times, have possessed so great a flexibility as is indicated by the movements we have referred to. Independently of that improbability, there are certain geological facts which are inexplicable on that assumption. Volcanic phenomena would be unintelligible; for vents traversing that thickness of solid rock could hardly be kept open owing to the cooling which the lava in its ascent would undergo. The rock fragments ejected during explosions are also those of rocks which lie at no great depth, while, with the increase of temperature in descending beneath the surface, there is every reason to suppose that at a depth to be measured by tens, and not by hundreds of miles, the immediate underlying magma at least is in a state of plasticity such as would allow of comparatively free movements of the crust. Again, surely, if the crust were so thick, we might expect to find, when that crust was broken and its edges thrust up by compression or protrusion of the igneous rocks, that some indications of that enormous thickness should be exhibited; but none such are forthcoming. Whatever may be the state of the nucleus, there is nothing geologically to indicate, as some physicists also have contended on other grounds, that the outer crust of the earth is more than from about twenty to thirty miles thick. The effective rigidity will therefore, if it be necessary, have to be explained in some other manner than that of a comparatively solid globe or of a crust of enormous thickness.
We are thus brought face to face with apparently irreconcilable opinions. That they admit of adjustment there can be no doubt, but it must be by mutual understanding. How it is to be effected is a problem for the future.
These, briefly, are the barriers which restrict inquiry on many important questions. On the side of the uniformitarians, it is assumed that every position must be reduced to a fixed measure—where fixity is not possible—of time and speed; and, on that of the physicists, geologists are gently reminded that the subject is outside their immediate sphere of inquiry, in a way somewhat suggestive of "the closure."
It would be an unfortunate day for any science to have free discussion and inquiry barred by assumed postulates, and not by the ordinary rules of evidence as established by the facts, however divergent the conclusions to which those facts lead may be from the prevailing belief. In any case it must be remembered that no hypothesis can be true which does not satisfy the conditions both of the geological phenomena and of the physical laws.