John George Francis

CONCERNING CERTAIN PETRIFIED ZOOPHYTES, NOW GENERALLY SUPPOSED TO BE EXTINCT.

The petrifaction of an animal form or structure has always appeared to me a most singular process in Nature. The change of such substances as cellular tissue, or fish-bone, or cartilage, or a light, leathery skin, into minerals such as silex and agate, seems at first like magic. Scott, when describing the “foliaged tracery” in the east oriel of Melrose Abbey, tells his reader:

But there is no magic in Nature; and when we meet with undoubted phenomena, however strange they may appear, in any department of her realm, we must behave ourselves like matter-of-fact persons and set about accounting for them logically. In a genuine petrifaction there are two ways of doing this, or, in other words, there are two different modes in which Nature may have acted so as to produce the result which we are considering. One of these is that of a simple substitution; the particles of one substance being gradually removed, and those of another taking their place. This must have been the process which obtained in many well-known instances: as, for example, when the thin, leathery shell or husk of an echinus is found perfectly rendered, both as to its general outline and minute markings, in solid flint or limestone. We feel quite sure that the original husk has perished; but here is its second self.

The other mode is that of actual transmutation, wherein no part of the original substance is destroyed or removed, but its physical conditions undergo a total change, by means of either infiltration, or crystallization, or perhaps both of them combined. This process has obtained in such cases as a petrified shark’s tooth, or a fossil trilobite from the Wenlock.

The tooth, which was once bone, is now a kind of metallic stone: the trilobite once, as we suppose, shell and cartilage, is now a something between limestone and cast-iron. And this change is very wonderful; but, as was said above, there is no magic in it, no more than in the hardening of an infant’s skull, or the solidifying of the arm and leg-bones by the absorption of phosphate of lime. Besides, the stone and metal now present are still something different from what we ordinarily mean by those terms: the petrified tooth is not like a flint-stone, neither could you cut horse-shoe nails out of the trilobite.

In fact, every real process has a character of its own: a something which distinguishes it from any other, despite of general resemblances up to a certain point.

Petrifaction, then, being assumed, as an operation which has really taken place after one way or another, it falls next to be considered, what was the original nature of these fossils? I mean the objects which we find thus preserved in our seaside pebbles. What was the department in Creation to which they belonged? Was it animal or vegetable before it thus became altogether mineral?

For myself, I will at once say that I have no doubt it was animal; but not an animal of a very high order. The specimens which I now possess, and which are all of them chosen, some of them rare, I should assign to such creatures as the “zoophytes” or “polyps,” both radiated and globular. And I know of nothing, among many hundreds of specimens gathered from a dozen different beaches, which presents the evidence of having belonged to vegetable organization, with the exception of sundry varieties of petrified wood, which speak for themselves, and could not be mistaken by anybody.

I will now state two or three reasons, which to my own mind are conclusive, for the above decision.

In the first place, then, the structures here perpetuated in stone are of great delicacy, and they have been immersed in ancient seas, as is testified by the localities in which they are obtained. Now vegetable structure as fine as this, if immersed long enough for any such change to come in question, must have utterly perished by maceration, and then the petrifaction could not have taken place. I know that part of the stem (and I think fruit) of one species of “conifer” has been found in the Isle of Wight in the condition of a fossil; but this belonged to a hardy class of plants, and the lobes or plates which composed its bark and husk are themselves highly siliceous, to say nothing of the presence of iron in the rind of most of these stems. So that the process would be a long one, and the fibrous material of the tree would stand it well. But in these pebbles some of the threads or tubes run from the size of small twine to that of the rays in a spider’s web; and no vegetable substance with which I am acquainted, excepting the filaments of “asbestos” (which is a vegetating mineral) in rock-crystal, could abide and retain its form, so as to allow of the changes by infiltration or otherwise which have passed upon the original structure.

If it be said in reply to this, that we have the exquisitely delicate “dendritic” markings, as of leaves and filaments of shrubs or sea-weed, in the heart of the white chalcedony “mocha-stones” from the East, the answer is evident: these are not really vegetable traces, but only resemble such in their configuration and colours. They are simply shoots and ramifications of a metal,—as iron or manganese. Those in the “weed-agate” of India, which exactly resemble sprays of fine sea-weed, are produced by “delessite.”

Indeed, it is both diverting and instructive to observe how Nature permits, and even seems to abound in, curious coincidences and striking resemblances between things of entirely diverse character. The dried polyp, called “encrinoid echinoderm,” bears a wonderful likeness to one species of Indian corn. (See the plate at p. 137 of Mr. Rymer Jones’s beautiful work on “The Animal Kingdom.”) And the other day, when I was enjoying a leisure hour in the British Museum, I suddenly remarked that the “carapace” (back-shell) of the splendid fossil specimen of the “Holoptychius nobilissimus” in one of the cases might serve for a sketch of the back of a capercailzie, where the grey and purple feathers overlap one another. Yet here is no real connection whatever. Only Dame Nature had gone to play.

Secondly. The preservation of these “polyp” forms, in the manner in which they have been preserved, seems to me to be due to a feature or circumstance which is strictly animal and not vegetable. I refer to the fact of the creature, while it was alive, inhabiting a house, a house built by himself, or emanating from his own substance. For, just as we could know but little of the existence or habits of “shell-fish,” were it not for their shells, so I think we may assume that the choanite must, when alive, have dwelt in a tough, horny “coperculum,” answering to the shape of his body and the number of his “polyps” (if he was a compound creature), because otherwise he would have been like a jelly-fish or naked slug, and his “polypary” could not have been preserved in a stony fossil. Of course, whatever was merely flesh, or adipose matter, has long since perished; but the house or shell in which it whilome dwelt remains. Thus, the “echinus” built himself a dome, such a residence as a hedgehog would require to live comfortably in, and through the various orifices of which his spines could be protruded at pleasure. The “ammonite,” being shaped like a snake, preferred living in a shell of that form, where the creature when coiled up was safe. The “alcyonite” had a more exquisite taste in house-building, answering, we may be sure, to the complex and beautiful structure with which the great Creator had endowed him. His home was a palace, containing long galleries and secret doors and wheel-windows; and here some of the delicate tubes are fringed at their extremities like the petals of a flower.

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.