The plants of the Wealden, so far as our knowledge extends at present, consist chiefly of Ferns, Coniferae (see fig. 244.), and Cycadeae, without any exogens; the whole more allied to the Oolitic than to the Cretaceous vegetation, although some of the species seem to be common to the chalk. But the vertebrate and invertebrate animals indicate, in like manner, a relationship to both these periods, though a nearer affinity to the Oolitic. Mr. Brodie has found the remains of beetles and several insects of the homopterous and trichopterous orders, some of which now live on plants, like those of the Wealden, while others hover over the surface of our present rivers. But no bones of mammalia have been met with among those of land-reptiles. Yet, as the reader will learn, in Chapter XX., that the relics of marsupial quadrupeds have been detected in still older beds, and, as it was so long before a single portion of the jaw of an iguanodon was met with in the Tilgate quarries (see p. 228.), we need by no means despair of discovering hereafter some evidence of the existence of warm-blooded quadrupeds at this era. It is, at least, too soon to infer, on mere negative evidence, that the mammalia were foreign to this fauna. In regard to the geographical extent of the Wealden, it cannot be accurately laid down; because so much of it is concealed beneath the newer marine formations. It has been traced about 200 English miles from west to east, from Lulworth Cove to near Boulogne, in France; and about 220 miles from north-west to south-east, from Whitchurch, in Buckinghamshire, to Beauvais, in France. If the formation be continuous throughout this space, which is very doubtful, it does not follow that the whole was contemporaneous; because, in all likelihood, the physical geography of the region underwent frequent change throughout the whole period, and the estuary may have altered its form, and even shifted its place. Dr. Dunker, of Cassel, and H. Von Meyer, in an excellent monograph on the Wealdens of Hanover and Westphalia, have shown that they correspond so closely, not only in their fossils, but also in their mineral characters, with the English series, that we can scarcely hesitate to refer the whole to one great delta. Even then, the magnitude of the deposit may not exceed that of many modern rivers. Thus, the delta of the Quorra or Niger, in Africa, stretches into the interior for more than 170 miles, and occupies, it is supposed, a space of more than 300 miles along the coast, thus forming a surface of more than 25,000 square miles, or equal to about one half of England.[237-A] Besides, we know not, in such cases, how far the fluviatile sediment and organic remains of the river and the land may be carried out from the coast, and spread over the bed of the sea. I have shown, when treating of the Mississippi, that a more ancient delta, including species of shells, such as now inhabit Louisiana, has been upraised, and made to