The antiquity of these formations may be very great, we can scarcely tell how far removed from our own era. For, while the zoophyte itself is of a perishable nature, we are acquainted with no substance more durable (if we except the gems) than that calcareous matter of which these tubes and plates were formed, when once it has been subjected to processes of infiltration by crystallizing mineral and metallic oxides.
Now, I think this argument a very strong one; in fact, although simple, almost unanswerable. For no plant dwells thus in a house. We have the plant itself, but nothing more; and if this be not capable, and I hold it to be incapable, of sustaining the most vehement mineralizing process in the crucible of Nature, its history must be a brief one, and excepting in the dark “lithographs” of the coal-measures, its memory must pass away. I have already allowed an exceptionable case in favour of the conifers, which, be it observed, nowise resemble anything portrayed in siliceous pebbles, and it is remarkable how much this class of plants predominates in the COAL.
I have always been suspicious of what are called “vegetable petrifactions.” I examined those at Tivoli, near Rome, in the year 1845, and I made up my mind that they are simple incrustations. In like manner, many of the buildings at Pæstum are constructed with a kind of “travertine” taken from the bed of a neighbouring river, and which rapidly incrusts any solid objects submitted to the action of its waters. But the truth is, the vegetable pipe or “straw” remains for a while, owing to the silex which entered into its composition while the plant was growing. After some years the straw decays, and there is a hole or depression in that part of the pillar or pediment. On the other hand, a calcareous “menstruum,” imbibing silex and iron, hardens into a substance which, like the best mortar or cement used in building, will sometimes outlast even the blocks of limestone or oolite which it was put to bind together.
Thirdly. Animal organization, such even as these polyps possessed, renders the phenomena much more intelligible. Our best authorities in such matters tell us that “insects have neither lungs nor branchiæ; but in them the air passes into a system of tubes, whose structure resembles that of an elastic webbing.” And again, “The annelids possess an uninterrupted circulation.” And again, “In the ‘Nymphon’ and ‘Pycnogonum’ molluscs, which are crustaceans having considerable resemblance to certain of our field spiders, the intestine penetrates to the very extremities of the feet and claws.”—Animal Kingdom. Now here are cited some of the very desiderata which I should have named, had I been asked what conditions were needed à priori for such petrifactions to occur. I will only add, under this head, that a fine annelid occurs in the blue agate off Eastbourne; that a “myriapod,” which is among the chromo plates of this volume, has all the characteristics of insect life and motion; and that a spider is the nearest thing I know of, in some respects, to what the “choanite” must have been when that mollusc condensed himself from a cylinder to a sphere. Perhaps, however, the strongest clause in this part of the argument may be drawn from the “sponges.” Here the creature itself, wonderful to relate, is a viscous fluid, and the intricate mansion which he inhabits is a globose, horny skeleton, perforated with endless small tubes opening into wider galleries. There was, however, in the perfect animal, I am assured, one main central cavity, which gave strength and unity to the entire fabric by the plan of its walls, and, perhaps, by a main valve. Throughout the whole of this hydraulic system the sea-water, on the circulation of which the zoophyte depended for life and health, could be pumped to and fro at pleasure. And, evidently, when the “habitat” of such a creature was suddenly invaded by a siliceous crystalline solution, extinction of the animal and a petrifying investment of his abode would be simultaneous.
Lastly. If the objects here petrified had been vegetable in their extraction, should we not, with the aid of the microscope, be able to identify them? But this I have never yet succeeded in doing; yet all the petrified “woods” are well known. I have myself obtained slabs of the “acacia” from the coast of South Devon; of the “beech,” in Sussex; of coniferous wood almost everywhere. And, what is more to the purpose, though the petrifaction in such cases is deep and perfect, no one looking upon it could doubt for a moment that the original structure had been that of wood from a forest-tree. Agatized as it is, and penetrated here and there by metallic colours, and shot with rays of jasper, the lines in its fabric reveal the texture of wood.
I may mention here, that every one who walks our beaches, with a view to the collection of fossil specimens, will do well to carry in his or her pocket a good lens, of large external diameter; mine measures about two inches across, and I may truly say it has saved me a world of trouble, besides affording me much satisfaction at odd moments in the scrutiny of pebbles of different kinds and textures.
Before closing this chapter I may be permitted to draw the reader’s attention to a theory held by the late Dr. Mantell. I cannot, at this moment, lay my hand upon the volume in which it occurs, but I am pretty sure it will be found in his “Geology of the Isle of Wight:” a book which, for elegance of composition, and sound information, can hardly be too much commended; though a resident lapidary in Sandown did once say to me, while thumbing the pages of a well-worn copy, “Ah, sir! if the Doctor had come here and stayed a week instead of listening only to what those fellows told him in Ryde, I could have shown him something which he doesn’t seem to know, as to how the bit of coast runs hereabouts.” Dr. Mantell’s idea was this: he held that when a mollusc was subjected to the first stage in the petrifying process, there was, in the dying of the creature, some effusion of blood (or quasi-blood), and that this, being the very pith and strength of the animal’s system, would, in many cases, tinge the future stone indelibly. He carried this notion so far as to assign some dark blotches, apparent in the masonry of a wall, to such a source as being their most probable cause; and he gave to the thing itself the graphic title of Molluskite. In this view I will only add that I am inclined to agree with him; and in my “myriapod,” already referred to, there is a blood-red spot which pierces through the stone, appearing on both sides, and which I at first supposed to be a piece of “shell-lac,” but I now rather regard it as the trace left of himself by some marauding “pholas,” who, after drilling a hole through the solid pebble, found his own grave there.