It is the coal series, which follows next in order, that is the great repository of plants. In this they are found in the shales, which are the substances best adapted for their preservation ; but they occur also in the limestone, the sandstone, and in the coal itself, as well as in the nodules of indurated clay, or argillaceous ironstone. We have already mentioned the manner in which the trees are found. The squamous trees are also found often in the sandstone, but not invariably ; as are fragments of wood, often turned into charcoal. But the more delicate plants are generally preserved in the shales ; while they also occur sometimes in the limestones, in unindurated clay, and in the ferruginous nodules. In many cases it is plain that these are aquatic plants, or such as have grown in moist woods ; and as they could not have undergone transportation, from the perfect manner in which they are preserved, we are very sure that they lie where they fell, and that these beds must therefore have been formed above the sea, and not beneath it.
The red marl stratum, which is a submarine one, has not yet been observed to contain vegetable remains, but they occur sparingly in the following limestone, known in England by the name of lias. It has not, however, been said that any vegetables have been found in any of the succeeding marine strata, even as high as the chalk, which is the last that is decidedly or solely of marine origin. But after this terminates, there occurs a great deposit of clay, which seems partly marine and partly terrestrial, and which, consequently, contains marine fossils, together with vegetable remains. In this bed, where it has been examined at Sheppey, it is said that 500 kinds of fruit have been found ; but it probably remains to be proved, whether some of these clays arc not rather the more recent alluvia of rivers than the old clay, and whether some of these cases are not analogous to that of Lincolnshire, already mentioned. Lastly, we must observe, that some vegeta ble remains of this nature occur in the fresh-water forma tions, and particularly in the upper deposit of the basin of Paris.
As we shall hereafter have occasion more particularly to remark, that the vegetables of the coal strata, which are those almost alone that merit description, are suppos ed to be, in their botanical habits, akin to the intertropical plants, we shall pass over this part of the subject, and merely point out the characters, as far as they can be as certained, of a few of the most remarkable. As yet, how ever, no extensive or accurate information of this kind can be procured ; because, till lately, no attempt has been made to describe or arrange them on general or natural principles, still less on generic ones, as has been done with respect to the fossil shells. But, as a work on this sub ject has just been commenced in Germany, we shall pro bably be soon able to distinguish and arrange them in a much better and more easy manner than heretofore; while, as the attention of collectors will thus also be more perfectly directed towards them, and to the discrimination of their characters, we can have no doubt that a great ac cession of species will be made to the yet scanty list.
From those revolutions of the face of the earth, which we are sure must have happened before the coal strata could have acquired their present positions, we ought to conclude that these plants could not be existing species. That is, these strata were formed above the waters of the ocean, and on dry land, whether in marshes or lakes, or both. But being covered by marine strata, they have sub sequently been overwhelmed by the ocean, and that for an immense period ; as we must conclude from the enor mous depth of that great deposit, which, in England, ex tends all the way upwards from the red marl to the chalk. Lastly, the ocean has again left them, or they have quitted it, so as to be now found in our hills and valleys far above its level. Now, under such changes of the_relative level of the land and sea as this, although marine animals might have gone on propagating their species, so that those now living and the imbedded ones may be the same, yet the vegetable creation most have been destroyed.
Thus we are not entitled to expect that the plants of the coal strata are the same as those now existing; and, ac cordingly, the most careful and acute botanists have never satisfied themselves about the identity of any one species, perhaps even one genus, although their natural orders, or prevailing affinities, can be made out. This, therefore, is all we can now expect, till future naturalists shall, (as we have just observed is doing) produce a system of subter ranean botany, in which such genera and species as are re quired may be created and named, while the whole are placed, as far as is possible, in orders or divisions corres ponding with those of the existing races.
Sonic specimens, sufficient to illustrate this subject in a manner adequate to the plan of this essay, having been given in our plates, we shall mention a few of the plants which have been examined by Sir James E. Smith, and other botanists of high reputation ; pointing out their analogies by using the names of living plants, without, however, meaning to insinuate that they belong either to these species or genera. With respect to the ferns, in deed, the modern generic characters are derived from minutia in the parts of fructification, such as to be with difficulty ascertained even in the living plants, and there fore utterly invisible in the fossil one. These, indeed, in all cases, even if alive, could not safely be distinguished by the mere inspection of the leaf, though the genus was known ; and in the fossil ones, of course, the difficulty is insuperable, because there are so many plants which they might resemble without being the same.