In the coal series, where the collision of marine and terrestrial bodies apears to be sometimes unquestionable, there does not seem much difficulty respecting the causes, taking, at least, a general view of the subject. Forests are now buried, and peat is formed in the marine estuaries of rivers; as cases similar to that described in Lincoln shire, in the first part of this article, are not uncommon ; and it is quite easy to conceive how marine remains should have been deposited in ancient beds of this nature, as they are at this day. There can be no doubt that the deposits which contain coal, have been made in this manner, or in one analogous; sometimes in inland lakes, and at others in estuaries. Thus the existence of marine remains in some of them becomes almost a necessary consequence. As to those cases where the mixture of these remains may be too intricate to admit of so simple a it must be recollected, that if there should seem to be any obscurity, it is necessary to be quite sure that the shells are of marine origin, and that the real position and rela tion of the different strata are well understood; and, lastly, that such cases, should they really prove complicated, may be referred to the following class of these appear ances, namely, to those in which marine and terrestrial strata alternate.
The next case is that of the alternating strata, of which the basin of Paris and that of the Isle of Wight may serve as examples; although they are also now known to exist in many other parts of the world. These deposits have been made after the uppermost of the marine strata, and they are at. the same time partial, or limited to a few separate spots ; being in this respect, indeed, not much unlike to the deposits of coal, which are equally partial and limited. As these collections of strata are not yet very numerous, and as they have not so far been compared as to admit of any general account which should apply to the whole, it will be necessary to give a sketch of that found near Paris, as being the most remarkable and the most thoroughly understood.
Here the uppermost secondary general marine stratum consists, as in England, of chalk, and it is within a cavity of it that the deposit in question is found. The total thickness of the whole is estimated at 500 feet; and in many places it is covered with a clay, occasionally inter mixed with strata of sand, and containing a few marine remains. This must be considered as the bottom of the basin ; and upon it there follows in order a collection of strata, which is considered to be divisible into four distinct series; the first and third being of marine origin, and the second and fourth having been deposited from fresh water. A mixed alluvial deposit covers the whole.
The first of these beds, or formations, consist, according to the geologists to whom we are indebted for the inf& mation, of several subordinate series of strata included within that which is called the lower marine limestone, and of a sandstone accompanied by a ciliceous limestone.
These subordinate series are divided into four, and consist of various alternations of limestone, marl, sandstone, chert, and shale; into the details of which it is here unnecessary to enter. They all contain marine shells; but the greater number, and the most numerous varieties, are included in the three first. In the fourth they are rare. It is further said, that some terrestrial and fresh-water shells occur in particular parts of the second and third series. The siliceous limestone contains no petrifactions. It is not necessary to enumerate the shells here, as a list of them is given in a more appropriate place.
The second series is the lowest fresh water formation, and consists of gypsum, intermixed with clay, and with calcareous and argillaceous marl. The beds include flint, rnenilite, and sulphate of strontian. The lowest parts seem to contain some marine shells; but in the principal part of the series are found fresh-water shells of various kinds, and fragments of palm trees, with remains of fishes and of several extinct quadrupeds, amphibious animals, and birds. The remains of the quadrupeds, it must be remarked, are found only in the upper beds of gypsum. It has already appeared that there are six genera of quad rupeds, of which three are known and three unknown. There is only one species of each of the three first, which are of the genera Canis, Viverra, and Didelphis. There is only one species, also, of one of the last genera, but the two others, distinguished by the names of Palxotherium and Anoplotherium, each contain four species. There are also three or four species of two of and a or crocodile.
The third series, or upper marine deposit, appears to commence with a bed of marl of various colours and qualities, followed by two beds of sandstone, and one of a cavernous quartz, of a chesty character, called millstone. As this last contains no shells of any kind, it may equally belong to this marine series, or to the fresh-water deposit that immediately succeeds. Neither does the lowest sandstone contain any shells ; but numerous species, to gether with the remains of fishes and crabs, arc found in the other strata.
The fourth and last series is a limestone of very va riable character, containing different flinty substances, and presenting appearances which need not be detailed. The shells which it contains appertain to genera that now exist in fresh water lakes and morasses in the same district ; and, besides these, it includes numerous remains of aquatic and other vegetables. It ought here to be remarked, that the fresh water deposits found in many other parts of France, in the Roman states, in Spain, and in Germany near Ulm and Mayence, are analogous chiefly to this upper deposit.