the valves of shells such as we know at the present day as living buried in the sand or mud of the sea-shore or of estuaries.  In other cases, the bed may obviously have been an ancient coral-reef, or an accumulation of social shells, like Oysters.  Lastly, if we find the deposit to contain the remains of marine shells, but that these are dwarfed of their fair proportions and distorted in figure, we may conclude that it was laid down in a brackish sea, such as the Baltic, in which the proper saltness was wanting, owing to its receiving an excessive supply of fresh water.

In the preceding, we have been dealing simply with the remains of aquatic animals, and we have seen that certain conclusions can be accurately reached by an examination of these.  As regards the determination of the conditions of deposition from the remains of aerial and terrestrial animals, or from plants, there is not such an absolute certainty.  The remains of land-animals would, of course, occur in “sub-aerial” deposits—­that is, in beds, like blown sand, accumulated upon the land.  Most of the remains of land-animals, however, are found in deposits which have been laid down in water, and they owe their present position to the fact that their former owners were drowned in rivers or lakes, or carried out to sea by streams.  Birds, Flying Reptiles, and Flying Mammals might also similarly find their way into aqueous deposits; but it is to be remembered that many birds and mammals habitually spend a great part of their time in the water, and that these might therefore be naturally expected to present themselves as fossils in Sedimentary Rocks.  Plants, again, even when undoubtedly such as must have grown on land, do not prove that the bed in which they occur was formed on land.  Many of the remains of plants known to us are extraneous to the bed in which they are now found, having reached their present site by falling into lakes or rivers, or being carried out to sea by floods or gales of wind.  There are, however, many cases in which plants have undoubtedly grown on the very spot where we now find them.  Thus it is now generally admitted that the great coal-fields of the Carboniferous age are the result of the growth in situ of the plants which compose coal, and that these grew on vast marshy or partially submerged tracts of level alluvial land.  We have, however, distinct evidence of old land-surfaces, both in the Coal-measures and in other cases (as, for instance, in the well-known “dirt-bed” of the Purbeck series).  When, for example, we find the erect stumps of trees standing at right angles to the surrounding strata, we know that the surface through which these send their roots was at one time the surface of the dry land, or, in other words, was an ancient soil (fig. 19).

[Illustration:  Fig. 19.—­Erect Tree containing Reptilian remains.  Coal-measures, Nova Scotia. (After Dawson.)