Thus we might conclude, that a certain degree of quity is necessary to the production of lignite, or that the process of bituminization is a work of time, though it may also, in particular situations, be aided by many collateral circumstances. The succession of strata at Bovey might lead to the opinion that successive forests had grown and been submerged in that particular spot ; an opinion which would be farther probable if time alone is the agent essen tial to the change, and seeing that the lowest strata are the most perfect. Yet we cannot be sure of this: as successive strata of wood and alluvium might thus have been depo sited by transportation.
There is great variety in the depth of the alternations in all these cases. The total depth at Bovey is said to be about 70 feet; this space, including all the coal, together with the alternating beds of clay and sand. The lowest or more perfectly bituminized stratum of the lignite, is sixteen feet thick. In the neighbourhood of Cologne, where the lignite is chiefly found in a tender or pulveru lent form, the depth is more considerable ; but the particu lar details of this deposit which have been given, do not lead to any valuable conclusions. Here the bed of lignite alone is said to be upwards of thirty feet thick. In Iceland it appears to be much more superficial, and occurs in the form of boards ; as if produced from the trunks of trees flattened by great pressure. This is the variety especial ly called surturbrand.
As jet is also found in similar situations, it follows, that in places not ostensibly different, all the varieties, from the most to the least bituminized, exist. Whether these differences argue a longer exposure to the producing caus es or not, we have not at present the means of ascertain ing. It is very difficult to form any conjecture respecting the relative dates of these deposits, as they regard the structure of the earth. It is, at any rate, certain, that they are far posterior, not only to coal, but even to the upper most secondary strata, although what rank they hold among the alluvia is not easily conjectured. Yet they seem to be prior to many of the trap rocks ; certainly to the latest, because the very fragments of wood found under these rocks belong to alluvial matters, and these, very probably, of the same age as the lignite deposits.
Their being limited to particular spots may have arisen from causes analogous to those which have equally limited the collections of animal remains in alluvia. If transport ed, they may have been determined by currents to particu lar valleys, or to lakes ; or if remaining in the places where they grew, they may have arisen from such forests as those of Lincolnshire. Considering the numerous examples of ancient alluvia that are known, it appears remarkable that the occurrence of lignites among these should be so rare.
It is difficult to conceive, that small portions only of the surface were covered with trees at the period of these al luvial formations, and equally difficult to comprehend by what means the deposition was limited to so small a num ber of places. The analogy of collections of animals does not assist us here ; as these are limited when alive, where as forests are confined to no place.
In rare cases, lignite is found under some secondary strata. That has been said to happen in different parts of France. Some of these rocks appear to be alluvial or fresh water strata ; and the coal series of Provence is altogether obscure. In the new discovered Melville island, on the north coast of America. lignite passing into coal is found under sandstone and limestone; both of which, however, seem to be of very recent origin. It is probable that these are of a date higher than the alluvial lignites, and, indeed, that they occur at many different periods in the later times of the earth's surface.
It is natural to ask, why lignites of a high antiquity do not put on the characters of true coal ? In the chemical character of jet, which is the most perfect, there is scarce ly any difference from coal. The real distinction of coal in this case is its mineral or mechanical character ; and it is easy to imagine how coal may have been placed, as we indeed know it has been, in circumstances in which lignite has not participated. But, indeed, as far as the chemical properties are concerned, many of the coals do resemble lignite in containing unchanged, or slightly changed wood ; and hence their bad smell in burning is often derived. These are, in fact, mixtures of lignite and coal.
The last geological situation in which lignite is found, is among the trap rocks, where it is sometimes known by the name of bituminized wood, or basaltic coal. In some cases, therefore, these have the characters of wood at one extreme, while at the other they are true coal; being also found in all the intermediate states. In some rare instan ces, also, the friable or pulverulent kind, resembling Co logne earth, is found in the same situations. Sometimes they are found in separate fragments, far asunder, or scat tered by chance; at others they are accumulated in partial spots, so as to form small irregular deposits. When these are more-considerable, the character of coal is sometimes perfect, and they have even been wrought for use. In Dr. Macculloch's account of the Western Islands, there is a drawing and description of a very considerable fragment of a tree found thus inclosed within trap.