There are some instances, however, in which the cir cumstances attending on these alluvial strata are such as to require additional notice. Italy, as offering the most remarkable examples of this nature, may be adduced as affording cases to illustrate this subject. Here, in some places, two alluvial strata, of decidedly distinct origin, are found together ; the one sufficiently distinguished by its containing the remains of marine bodies, and the other in cluding those of land animals. This peculiarity is explain ed by conceiving that this country has been elevated from the bottom of the sea, as Santorini and many other islands have been, by the force of volcanic fires. Thus the marine alluvia, raised above the surface of the waters in their na tural or unconsolidated state, have been immediately suc ceeded by terrestrial ones ; and thus the two classes of or game bodies, as distinct in their original seats as in their periods of deposition, become approximated.
In the same country, anomalies of another nature are produced by the calcareous rocks which are deposited in so many places. The travertino of Rome is of this nature ; and, from the mode of its formation, it often contains fresh-water shells, and the fragments of plants. Such rocks then lie above the marine alluvia in some instances ; and, in others, above both these and the terrestial ones, producing singular appearances, which for a long time were not understood, and which gave rise to a great deal of confusion and misconception.
But there is yet a case of rocks including organic re mains, which is independent of all these, and which par takes, in some measure, both of the nature of alluvia and of solid rock, if not of strata. These are the instances of which Dalmatia and Gibraltar afford examples ; in which the remains are found in cavities, or irregular masses, that appear to have occupied fissures, and which may, in a certain sense, be looked on as veins. Such rocks consist chiefly of calcareous fragments, intermixed with the re mains in question, and consolidated into a mass by sand, smaller fragments, and carbonate of lime that has been in solution in water. Such organic remains are, as might be expected, of terrestrial origin, and are generally of dates corresponding with those of the alluvial soils.
These may therefore be considered, in some sense, as belonging to the class of alluvial rocks, and as ranking, therefore, as far as their age is concerned, with the traver tinos and other similar substances. And, under this head, we may also include those recent rocks formed principally from calcareous fragments, which are occasionally found on sea shores. Not to accumulate unnecessary examples, the coast near Messina is remarkable for rocks of this nature ; so large in quantity, and so solid, as to be quarri ed for millstones Such rocks are, in every thing but age, the same as the Purbeck stone of England. In the West
Indies, as already mentioned, they also occur; and here they have, in one instance, attracted great attention ; in that, namely, of Guadaloupe, where human skeletons were thus found imbedded in a calcareous rock, somewhat re sembling both chalk and oolite.
We must not, however, even yet, dismiss the alluvial substances that contain organic remains, without advert ing to those remarkable instances where they are found in caves ; in which cases they either may or may not be surrounded by various earths, more or less perfect, and more or less consolidated. The caves of Bayreuth, which will hereafter be mentioned, present an example of this nature ; and, in our own country, various bones or skele tons of large animals have been found in similar situations, as in Yorkshire, and at Plymouth ; in which latter instance, the cavity included clay, together with the bones of the rhinoceros, the rocks themselves being limestone.
Ice cannot with propriety be called a rock or a stratum ; but, as far as it is a repository of ancient and lost animals, it may be considered an alluvial deposit, and requires to be mentioned here among the substances which have pre served such remains for our inspection. In the northern parts of Asia, it has been found to inclose the remains of some of the larger lost quadrupeds ; and, what renders it particularly conspicuous in this respect, the flesh itself was so preserved, that the whole animal, and not the skeleton only, was capable of being examined.
It is yet necessary, before concluding this enumeration of the substances in which organic remains are found im bedded, to mention such as cannot be considered as rocks, but minerals. Coal may rank with either, according to the views we choose to take of it ; and it is, as is well known, the occasional repository of what may be consider ed an organic subStance, namely, charred wood. Under the head of unquestionable minerals must be ranked flint, which often contains shells within its nodule, while it also penetrates these bodies in different modes. To this we must also add chalcedony, sometimes called agate. If, in some cases, the chalcedonies are rather the contained than the containing substance, as in the common example of silicified wood, there are others in which the organic body is truly imbedded in the mineral. This is the peculiar case of the mosses inclosed in agates, which will be de scribed hereafter in their proper place. Lastly, we must add amber, which, although a rare instance, cannot with propriety be omitted ; as it is nearly the only substance which has preserved for our inspection specimens of the insect creation.