Hooke discusses the similarities between petrified wood (i.e. coal) and charcoal, and suggests how fossils may have come to exist. Discusses ammonites.
That which I more particular examin’d, was a piece about the bigness of a mans hand, which seem’d to have been a part of some large tree, that by rottenness had been broken off from it before it began to be petrify’d.
And indeed, all that I have yet seen, seem to have been rotten Wood before the petrifaction was begun; and not long since, examining and viewing a huge great Oak, that seem’d with meer age to be rotten as it stood, I was very much confirm’d in this opinion; for I found, that the grain, colour, and shape of the Wood, was exactly like this petrify’d substance; and with a Microscope, I found, that all those Microscopical pores, which in sappy or firm and sound Wood are fill’d with the natural or innate juices of those Vegetables, in this they were all empty, like those of Vegetables charr’d; but with this difference, that they seem’d much larger then I have seen any in Char-coals; nay, even then those of Coals made of great blocks of Timber, which are commonly call’d Old-coals. […]
This Petrify’d substance resembled Wood, in that
First, all the parts of it seem’d not at all dislocated, or alter’d from their natural Position, whil’st they were Wood, but the whole piece retain’d the exact shape of Wood, having many of the conspicuous pores of wood still remaining pores, and shewing a manifest difference visible enough between the grain of the Wood and that of the bark, especially when any side of it was cut smooth and polite; for then it appear’d to have a very lovely grain, like that of some curious close Wood.
Next (it resembled Wood) in that all the smaller and (if I may so call those which are onely visible with a good magnifying Glass) Microscopical pores of it appear (both when the substance is cut and polish’d transversly and parallel to the pores of it) perfectly like the Microscopical pores of several kinds of Wood, especially like and equal to those of several sorts of rotten Wood which I have since observ’d, retaining both the shape, position and magnitude of such pores. […]
That petrify’d Wood having lain in some place where it was well soak’d with petrifying water (that is, such a water as is well impregnated with stony and earthy particles) did by degrees separate, either by straining and filtration, or perhaps, by precipitation, cohesion or coagulation, abundance of stony particles from the permeating water, which stony particles, being by means of the fluid vehicle convey’d, not onely into the Microscopical pores, and so perfectly stoping them up, but also into the pores or interstitia, which may, perhaps, be even in the texture or Schematisme of that part of the Wood, which, through the Microscope, appears most solid, do thereby so augment the weight of the Wood, as to make it above three times heavier then water, and perhaps, six times as heavie as it was when Wood.
By this intrusion of the petrifying particles, this substance also becomes hard and friable; for the smaller pores of the Wood being perfectly wedg’d, and stuft up with those stony particles, the small parts of the Wood have no places or pores into which they may slide upon bending, and consequently little or no flexion or yielding at all can be caus’d in such a substance.
Observations of serpentine stones (ammonites)
Nor is Wood the onely substance that may by this kind of transmutation be chang’d into stone […]. [I…] set down some Observation I lately made on several kind of petrify’d Shels, found about Keinsham, which lies within four or five miles of Bristol, which are commonly call’d Serpentine-stones.
Examining several of these very curiously figur’d bodies (which are commonly thought to be Stones form’d by some extraordinary Plastick virtue latent in the Earth itself) I took notice of these particulars:
[T]hey were very different, as to their outward covering, some having the perfect Shell, both in figure, colour, and substance, sticking on upon its surface, and adhering to it, but might very easily be separated from it, and like other common Cockle or Scolop-shels, which some of them most accurately resembled, were very dissoluble in common Vinegar, others of them, especially those Serpentine, or Helical stones were cover’d or retained the shining or Pearl-colour’d substance of the inside of a Shel, which substance, on some parts of them, was exceeding thin, and might very easily be rubbed off; on other parts it was pretty thick, and retained a white coat, or flaky substance on the top, just like the outsides of such Shells; some of them had very large pieces of the Shell very plainly sticking on to them, which were easily to be broken or flaked off by degrees: they likewise, some of them retain’d all along the surface of them very pretty kind of sutures, such as are observ’d in the skulls of several kinds of living creatures, which sutures were most curiously shap’d in the manner of leaves, and every one of them in the same Shell, exactly one like another, which I was able to discover plainly enough with my naked eye, but more perfectly and distinctly with my Microscope; all these sutures, by breaking some of these stones, I found to be the termini, or boundings of certain diaphragms, or partitions, which seem’d to divide the cavity of the Shell into a multitude of very proportionate and regular cells or caverns, these Diaphragms, in many of them, I found very perfect and compleat, of a very distinct substance from that which fill’d the cavities, and exactly of the same kind with that which covered the outside, being for the most part whitish, or mother-of-pearl colour’d.
Hooke’s conclusions on fossils
From all which, and several other particulars which I observ’d, I cannot but think, that all these, and most other kinds of stony bodies which are found thus strangely figured, do owe their formation and figuration, not to any kind of Plastick virtue inherent in the earth, but to the Shells of certain Shel-fishes, which, either by some Deluge, Inundation, Earthquake, or some such other means, came to be thrown to that place, and there to be fill’d with some kind of Mudd […], which in tract of time has been settled together and hardned in those shelly moulds into those shaped substances we now find them; […] that these Shells which are thus spirallied and separated with Diaphragmes, were some kind of Nautili or Porcelane shells; and that others were shells of Cockles, Muscles, Periwincles, Scolops, &c. of various sorts; that these Shells in many, from the particular nature of the containing or enclos’d Earth, or some other cause, have in tract of time rotted and mouldred away, and onely left their impressions, both on the containing and contained substances; and so left them pretty loose one within another, so that they may be easily separated by a knock or two of a Hammer.
For it seems to me quite contrary to the infinite prudence of Nature, which is observable in all its works and productions, to design every thing to a determinate end, and for the attaining of that end, makes use of such ways as are (as farr as the knowledge of man has yet been able to reach) altogether consonant, and most agreeable to man’s reason, and of no way or means that does contradict […]; whence it has a long time been a general observation and maxime, that Nature does nothing in vain; It seems, I say, contrary to that great Wisdom of Nature, that these prettily shap’d bodies should have all those curious Figures and contrivances (which many of them are adorn’d and contriv’d with) generated or wrought by a Plastick virtue, for no higher end, then onely to exhibite such a form […]