of dead vegetable matter. By their weight they compressed it, and by certain chemical changes (which we have not time to go into this evening) this dense mass of vegetable matter became converted into coal. After a time the shales and sandstones which had been piled above this stuff, which was to form coal for the future, were again elevated to form a land surface; upon this another forest sprang up, and by its decay produced another mass of vegetable matter fit to form coal. This again was let down below the water, more shales and sandstones were deposited on the top, and this process went on over and over again till the whole mass of our present coal measures was formed. You will now see how it is that trees are so seldom found in an upright position in the coal beds. As the land went down, they would in very many cases be toppled over by the water as it flowed against them, or their base would be rotted, and they would then either fall or be blown over; that is the reason why in most cases they are found lying flat on the roof of the coal bed. But in a few cases, when the depression was very gentle and gradual, the trees were not overthrown, and the shales and sandstones accumulated round them and preserved them in the position in which they grew. I do not know that I can point out to you anything nowadays that exactly resembles the state of things that must have gone on during the times these coal measures were being formed; but there are a great many cases strikingly analogous to them. I shall not attempt to describe them to you, but may just mention the mangrove swamps that very often fringe the coasts in the tropics, and the cypress swamps of the Mississippi, which are so well described by Sir Charles Lyell in his recent works; also the great Dismal Swamp of Virginia, which appears to me to furnish the nearest analogue to the state of things that existed during coal measure times.