The circumstances in which the first specimens of Carboniferous snails and gally-worms were found are so peculiar and so characteristic of the coal formation, that I must pause here to notice them, and to make of them an introduction to the next group of creatures we have to consider. In the coal formation in all parts of the world it is not unusual, as stated already in a previous page, to find erect trees or stumps of trees, usually Sigillariæ, standing where they grew; and where the beds are exposed in coast cliffs, or road cuttings, or mines, these fossil trees can be extracted from the matrix and examined. They usually consist of an outer cylinder of coal representing the outer bark, while the space within, once occupied by the inner bark and wood, is filled with sandstone, sometimes roughly arranged in layers, the lowest of which is usually mixed with coaly matter or mineral charcoal derived from the fallen remains of the decayed wood, a kind of deposit which affords to the fossil botanist one of the best modes of investigating the tissues of these trees. These fossil stumps are not uncommon in the roofs of the coal-seams. In some places they are known to the miners as "coal pipes," and are dreaded by them in consequence of the accidents which occur from their suddenly falling after the coal which supported them has been removed. An old friend and helper of mine in Carboniferous explorations had a lively remembrance of the fact that one of these old trees, falling into the mine in which he was working, had crushed his leg and given him a limp for life; and if he had been a few inches nearer to it would have broken his back.
The manner in which such trees become fossilized may be explained as follows:--Imagine a forest of Sigillariæ growing on a low flat. This becomes submerged by subsidence or inundation, the soil is buried under several feet of sand or mud, and the trees killed by this agency stand up as bare and lifeless trunks. The waters subside, and the trees rapidly decay, the larvæ of wood-boring insects perhaps aiding in the process, as they now do in the American woods. The dense coaly outer bark alone resists decomposition, and stands as a hollow cylinder until prostrated by the wind or by the waters of another inundation, while perhaps a second forest or jungle has sprung up on the new surface. When it falls, the part buried in the soil becomes an open hole, with a heap of shreds of wood and bark in the bottom. Such a place becomes a fit retreat for gally-worms and land-snails; and reptiles pursuing such animals, or pursued by their own enemies, or heedlessly scrambling among the fallen trunks, may easily fall into such holes and remain as prisoners. I remember to have observed, when a boy, a row of post-holes dug across a pasture-field and left open for a few days, and that in almost every hole one or two toads were prisoners. This was the fate which must have often befallen the smaller reptiles of the coal forests in the natural post-holes left by the decay of the Sigillariæ. Yet it may be readily understood that the combination of circumstances which would effect this result must have been rare, and consequently this curious fact has been as yet observed only in the coal formation of Nova Scotia; and in it only in one locality, and in this in one only out of more then sixty beds in which erect trees have been found. But these hollow trees must be filled up in order to preserve their contents; and as inundation and subsequent decay have been the grave-diggers for the reptiles, so inundations filled up their graves with sand, to be subsequently hardened into sandstone, burying up at the same time the newer vegetation which had grown upon the former surface. The idea that something interesting might be found in these erect stumps, first occurred to Sir C. Lyell and the writer while exploring the beautiful coast cliffs of Western Nova Scotia in 1851; and it was in examining the fragments scattered on the beach that we found the bones of the first Carboniferous reptile discovered in America, and the shell of the oldest known land snail.