The soil is a sofa clay beneath, but the surface is entirely composed of rotten leaves, among which were found those of the holly and of common reeds. At Sutton, at the depth of sixteen feet, the same soil was found on digging, as it. was also in some other plaLc3 which we need not here specify. Now, the extent and uniform level of this sur face, and the appearance of these trees, which must have grown where they now are, are sufficient to prove that this land, now below the level of the sea, was once above it ; and as we have no reason to think that the sea has risen, it is concluded that the land must have subsided. On the opposite side of the Channel, Holland presents similar ap pearances.
On Silicified and other Petrified Woods.
Wood, more or less perfectly penetrated with siliceous matter, has been found in various parts of the world. The trunks of palm trees have been thus observed in many parts of Africa; and General Regnier procured one of this nature from the isthmus of Suez. According to Horneman, it occurs very abundantly in the desert, on the confines of Egypt, in various modes. Sometimes whole trunks are found, of twelve feet and more in circumfer ence; sometimes fragments and branches, and at others pieces of bark. Among these, he says, he observed the oak. Many_ of the great stems still retain their side branches, and in some the natural state of the wood is so little altered, that the fibrous structure can be distinctly traced. Sometimes the inner part of the tree was petri fied, while the outside was in its natural state. He was also informed by the Arabs that such petrified trees were often found growing upright. The wood was black or grey ; and sometimes these substances were found dis persed, while at others they occupied beds or strata.
On the shores near Madras, silicified wood is very abundant, and it is found in considerable fragments or trunks. It is very perfectly converted into stone, but still retains all the marks of the vegetable structure ; being of a red and white mottled colour, it is frequently cut into beads and other ornaments. It is known by the name of tamarind tree by the English ; but the tamarind does not grow in that neighbourhood.
Antigua also produces great quantities of silicified wood, as well as corals, and other marine productions, pre served in the same manner. The woods are various in aspect, and often very beautiful in the transverse sections.
One kind appears to have been of a very open and soft texture, but it is not conjectured to what tree it belongs. Silicified wood occurs in Loughneagli, in Ireland, as al ready remarked, and also in various parts of Germany, as well as in our own country, in many places. But it is unnecessary to dwell on these localities of a substance so common. The woods that are occupied, rather than pe trified by sandstone, and which occur among the coal strata, require a separate consideration.
The manner in which this change is produced has not been satisfactorily explained, though there is no want of theories on the subject. There is no question, in the first place, that silica is held in solution in many waters, and that it even can be suspended in watery vapour ; as in this manner siliceous concretions are known to be formed in the volcanoes of Italy. The siliceous incrustations of the hot springs of Iceland and the Azores, are well known. It is also owing to the same cause that chalce donies, whether in the shape of stalactites, or solid no dules, are formed in the trap rocks. Thus the agent is found, but the difficulty remains the same respecting the mode. Very often not only is the whole substance of the vegetable entirely destroyed, but the texture, all the most minute organization of the vessels, is so perfectly preserved, that the transverse sections of the stone resemble those which are made from the wood itself for microscopic pur poses. The difficulty here is to explain how the substitution of one part for the other could have taken place, while it must at the same time have been as gradual as it is com plete. Mr. Kirwin's theory of substitution explains no thing; and Mr. Parkinson's supposition that the process is circuitous, the wood being first bituminized and then silicified, is much worse ; as the first effect of bituminiza tion, is either to destroy all the fine and intricate texture, or else to fill the intervals with bitumen. In Dr. Hutton's theory, it is imagined that the siliceous matter has been introduced by fusion ; a supposition which is at variance with all facts and all chemistry ; and which, it is not too much to say, is even at variance with all possibility. It is better for the present to remain content with our igno rance, than to invent unintelligible hypotheses.