Of Fossil Colonies.
It is an important part of the history of fossils, for the purpose of geological science, and for the sake of the in fcrences to which this may lead, to inquire in what man ner they occur in the strata. The mode in which many shells are found in distinct colonies, is no less remarkable than their numbers. It it far, however, from being an in variable fact, that particular tribes occur alone; but this happens sufficiently often to prove that they are now in the very state in which they lived and died, although the whole mass may be far removed from its original situation. The same fact is confirmed by their frequent integrity, and by the peculiar and regular modes in which they lie in the strata, relatively to the planes of the stratification.
Of those which thus form distinct colonies, none are more remarkable than the nummulites, the ammonites, and the cerithii, which have all been observed to produce masses of rock of great extent and depth. Some of the ammonites and nautili thus found, are so minute as to be almost microscopic objects; it having been observed in Italy that upwards of 10,000, when collected from a loose calcareous sandstone, did not weigh 180 grains. One or both of these genera, as they are not always disting uishable, appear to have been very abundant in the an cient globe, as some hundred species have been ascertain agd, varying in size from the diameter just mentioned to one of four or five feet. The enormous collections of shells that exist in France, in Touraine, and at Grignon, have attracted much notice. The former is said to co ver a space of nine square leagues, with a mean depth of eighteen feet ; the whole of this immense mass consist ing entirely of fossil shells. It is said that at Grignon, while the quantity is also very great, 600 species and upwards have been discovered. But such facts as these have been observed in many parts of the world ever since the days of Herodotus.
Whether, lastly, these animals occur in separate co lonies or more intermixed, their immense numbers in one place, and their existence in successive strata of rock, no less than the changes of species and genera which these strata present, indicate the slow succession in which these rocks were formed. Ages must have been required for the reproduction and death of animals so innumerable, however prolific they must be supposed ; and ages must alike have passed before the loose materials in which they are entangled, or by which they are covered, could have been accumulated on the bottom of the sea. If there
were no other arguments sufficient to prove that rocks were not formed either by a sudden or slow precipitation of earthy matter, from a state of solution in water, the mere history of these fossil collections would be sufficient to prove their mechanical and tedious production.
On the Successions of Organic Fossils.
As a general, if not an absolute and invariable order of succession among rocky strata has been discovered, and as, in certain cases, or districts, that order has been found, under certain modifications, to be regular, it has been supposed that some similar or analogous order existed among the places, numbers, and tribes, of the fossil bodies which they contain. Two points of some importance are connected with this opinion ; the one relating to an absolute order in the creation or existence of the different races of organized beings, and the other to the invariable or necessary connections of particular species or genera with corresponding strata. The latter question must be reserved for a separate examination at the end of this article, where it will be better understood : the other, as far as it admits of any illustration, may be examined after the general question has been discussed.
To examine, in the most general manner, the succes sion of organized bodies merely as they exist in the earth, it must first be remarked, that wherever we may choose to commence among the upper strata, there is not an uninterrupted succession of them downwards to the point at which they cease to be found. This might, indeed, already have been deduced, from the the remarks which have been made on the nature of the strata in which they do and do not exist; but it is requisite that it should be stated here for the sake of the present inquiry. Many strata abound in them, when they are wanting in those immediately above or below ; although it is still a gen eral rule, that they are most numerous and various in the upper beds, and that, at a certain point downwards, they either materially diminish or entirely disappear. But as it was already shown that these bodies were chiefly pre valent in the secondary strata, and, among these, princi pally in the shales and limestones, and that they never occurred in the unstratified rocks, this part of the sub ject has been necessarily anticipated.