Thus the low-lying parts of the mighty city of London became swamps, and the higher grounds were clad with bushes. The very largest of the buildings fell in, and there was nothing visible but trees and hawthorns on the upper lands, and willows, flags, reeds, and rushes on the lower. These crumbling ruins still more choked the stream, and almost, if not quite, turned it back. If any water ooze past, it is not perceptible, and there is no channel through to the salt ocean. It is a vast stagnant swamp, which no man dare enter, since death would be his inevitable fate. There exhales from this oozy mass so fatal a vapour that no animal can endure it. The black water bears a greenish-brown floating scum, which for ever bubbles up from the putrid mud of the bottom. When the wind collects the miasma, and, as it were, presses it together, it becomes visible as a low cloud which hangs over the place. The cloud does not advance beyond the limit of the marsh, seeming to stay there by some constant attraction; and well it is for us that it does not, since at such times when the vapour is thickest, the very wildfowl leave the reeds, and fly from the poison. There are no fishes, neither can eels exist in the mud, nor even newts. It is dead. The flags and reeds are coated with slime and noisome to the touch; there is one place where even these do not grow, and where there is nothing but an oily liquid, green and rank. It is plain there are no fishes in the water, for herons do not go thither, nor the kingfishers, not one of which approaches the spot. They say the sun is sometimes hidden by the vapour when it is thickest, but I do not see how any can tell this, since they could not enter the cloud, as to breathe it when collected by the wind is immediately fatal. For all the rottenness of a thousand years and of many hundred millions of human beings is there festering under the stagnant water, which has sunk down into and penetrated the earth, and floated up to the surface the contents of the buried cloacæ. Many scores of men have, I fear, perished in the attempt to enter this fearful place, carried on by their desire of gain. For it can scarcely be disputed that untold treasures lie hidden therein, but guarded by terrors greater than fiery serpents. These have usually made their endeavours to enter in severe and continued frost, or in the height of a drought. Frost diminishes the power of the vapour, and the marshes can then, too, be partially traversed, for there is no channel for a boat. But the moment anything be moved, whether it be a bush, or a willow, even a flag, if the ice be broken, the pestilence rises yet stronger. Besides which, there are portions which never freeze, and which may be approached unawares, or a turn of the wind may drift the gas towards the explorer.