THE indelible records of past events which are preserved within the solid substance of our globe, may be in some measure understood without the aid of that refined analysis on which a complete acquaintance with them depends. The remains of vegetation, and of animal life, embedded in their coeval rocks, attest the existence of far distant times; and as science and the arts advance, we shall be enabled to read the minuter details of their living history. The object of the present note is to suggest to the reader a line of inquiry, by which we may still trace some small portion of the history of the past in the fossil woods which occur in so many of our strata.
It is well known that dicotyledonous trees increase in size by the deposition of an additional layer annually [256/257] between the wood and the bark, and that a transverse section of such trees presents a series of nearly concentric though irregular rings, the number of which indicates the age of the tree. The relative thickness of these rings depends on the more or less flourishing state of the plant during the years in which they were formed. Each ring may, in some trees, be observed to be subdivided into others, thus indicating successive periods of the same year during which its vegetation was advanced or checked. These rings are disturbed in certain parts by irregularities resulting from branches; and the year in which each branch first sprung from the parent stock may be ascertained by proper sections.
It has been found by experiment, that even the motion imparted to a tree by the winds has an influence on its growth. Two young trees of equal size and vigour were selected and planted in similar circumstances, except that one was restrained from having any motion in the direction of the meridian, by two strong ropes fixed to it, and connecting it to the ground, at some distance towards the north and south. The other tree was by similar means prevented from having any motion in the direction of east and west. After several years, both trees were cut down, and the sections of their stems were found to be oval; but the longer axis of the oval of each was in the direction in which it had been capable of being moved by the winds.
These prominent effects are obvious to our senses, but every shower that falls, every changeof temperature that occurs, and every wind that blows, leaves on the vegetable world the traces of its passage; slight, indeed, and imperceptible, perhaps, to us, but not the less permanently recorded in the depths of those woody fabrics. All these indications of the growth of the living tree are preserved in [240/241] the fossil trunk, and with them also frequently the history of its partial decay.
Let us now inquire into the use we may make of these details relative to individual trees, when examining forests submerged by seas, embedded in peat mosses, or transformed, as in some of the older strata, into stone. Let us imagine, that we possessed sections of the trunks of a considerable number of trees, such as those occurring in the bed called the Dirt-bed1 in the island of Portland. If we were to select a number of trees of about the same size, we should probably find many of them to have been contemporaries. This fact would be rendered probable if we observed, as we doubtless should do, on examining the annual rings, that some of them conspicuous for their size occurred at the same distances of years in several trees. If, for ex- ample, we found on several trees a remarkably large annual ring, followed at the distance of seven years by a remarka- bly thin ring, and this again, after two years, followed by another large ring, wĘ should reasonably infer that seven years after a season highly favourable to the growth of these trees, there had occurred a season peculiarly unfavourable to them; that after twomoreyears anotherveryfavourable season had happened, and that all the trees so observed had existed at the same period of time. The nature of the season, [242/243] whether hot or cold, wet or dry, might be conjectured with some degree of probability, from the class of tree under consideration. This kind of evidence, though slight at first, receives additional and great confirmation by the discovery of every new ling which supports it; and, by an extensive con- currence of such observations, the succession of seasons might be in some measure ascertained at remote geological periods.
On examining the shape of the sections of such trees, we might perceive some general tendency towards a uniform inequality in their diameters; and we should perhaps find that the longer axes of the sections most frequently pointed in one direction. If we knew from the species of tree that it possessed no natural tendency to snch an inequality, then we might infer that, during the growth of these trees, they were bent most frequently in one direction; and hence derive an indication of the prevailing winds at that time. In order to determine from which of the two opposite quarters these winds came, we might observe the centres of these sections; and we should generally find that the rings on one side were closer and more compressed than those on the opposite side. From this we might infer the most exposed side, that from which the wind most frequently blew. Doubtless there would be many exceptions arising from local circumstances — some trees might have been sheltered from the direct course of the wind, and have only been acted upon by an eddy. Some might have been protected by adjacent large trees, sufficiently near to sliclter them from the ruder gales, but not close enough to obstruct the light and [242/243] air by which they were nourished. Such a tree might have a series of large and rather uniform rings, during the period of its protection by its neighbour; and these might be followed by a series of stinted and irregular ones, occasioned by the destruction of its protector. The same storm might have mutilated some trees, and half uprooted others: these latter might strive to support themselves for years, making but little addition, by stinted layers, to the thickness of their stems; and then, having thrown out new roots, they might regain their former rate of growth, until a new tempest again shook them from their places. Similar effects might result from floods and tlie action of rivers on the trees adjacent to their banks. But llie effect of all these local and peculiar circumstances would disappear, if a sutEcient number of sections could be procured from fossil trees, spread over a considerable extent of country.
The annual rings might however furnish other intimations of the successive existence of the trees..
On examining some rings remarkable for their size and position, let us suppose that we find, in one section, two remarkably large rings, separated from another large ring, by one very stinted ring, and this followed, after three ordinary rings, by two very small and two very large ones. Such a group might be indicated by the letters —
where o denotes an ordinary year or ring, L a large one, and s a small or stinted ring. If such a group occurred in [243/244] the sections of several different trees, it might,fairly be attributed to general causes.
Let us now suppose such a group to be found near the centre of one tree, and towards the external edge or bark of another; we should certainly conclude, that the tree near whose bark it occurred was the more ancient tree; that it had been advanced in age when that group of seasons occurred which had left their mark near the pith of the more recent tree, which was young at the time those seasons happened. If, on counting the rings of this younger tree, we found that there were, counting inward from the bark to this remarkable group, three hundred and fifty rings, we should justly conclude that, three hundred and fifty years before the death of this tree, which we will call A, the other, which we will call B, and whose section we possess, had then been an old. tree. If we now search towards the centre of the second tree B, for another remarkable group of rings; and if we also find a similar group near the bark of a third tree, which we will call C; and if, on counting the distance of the second group from the first in B, we find an interval of 430 rings, then we draw the inference that the tree A, 350 years before its destruction, was influenced in its growth by a succession of ten remarkable seasons, which also had their effect on a neighbouring tree B, which was at that time of a considerable age. We conclude farther, that the tree B was influenced in its youth, or 420 years before the group of the ten seasons, by another remarkable succession of seasons, which also acted on a third tree, C, then old. Thus we connect the time of the death of the tree A with the series of seasons which affected the tree in its old age, at a period 770 years antecedent. If we iould discover other trees having other cycles of seasons, capable of identification, we might trace back the history of the ancient forest, and possibly find in it some indications for conjecturing the time occupied in forming the stratum in which it is embedded.
The application of these principles to ascertaining the age of submerged forests, or to that of peat mosses, may possibly connect them ultimately with the chronology of man. Already we have an instance of a wooden hut with a stone hearth before it, and burnt wood on it, and a gate leading to a pile of wood, discovered at a depth of fifteen feet below tlie surface of a bog in Ireland: and it was found that this hut had probably been built when the bog had only reached lialf its present thickness, since there were still fifteen feet of turf below it.
The realization of the views here thrown out would require the united exertions of many individuals patiently exerted through a series of years. The first step must be to study fully the relations of the annual rings in every part of an individual tree. The effect of a favourable or unfavourable season on a section near the root must be compared with the influence of the same circumstance on its growth towards the top of the tree. Vertical sections also must be examined in order to register the annual additions to its height, and to compare them with its increase of thickness. Every branch, must be cect to its origin, and its sections be registered. The means of identifying the influence of differt seasons in various: sections of the same individual tree an its branches being thus attained, the conclusions arrived at must be applied to several trees under similar circumstances, and such modifications must be applied to them as the case may require; and before any general conclusions can be reached respecting a tract of country once occupied by a forest, it will be necessary to have a considerable number of sections of trees scattered over various parts of it.
14 December 2008