Lying immediately between the oolite and the chalk, is a small formation of fresh water, and not of marine deposit, to which the term Wealden has been given. This name has been given to it because it has been found developed chiefly in the Wealds, or Wolds,[98] of Sussex, Surrey, and Kent; and to Dr. Mantell belongs the honour of investigating this singularly interesting formation, and of giving us its history, after months of patient research and laborious toil conducted on the spot, just as Hugh Miller has become the historian and explorer of the Old Red Sandstone of Cromarty. But although the Wealden is small when compared with the vast extent of some other formations, previously or subsequently added to the material of this “great globe’s” crust, it possesses unusual interest on account of the strange organic remains that are found in it, and of the evidence which these remains supply of vast changes in the conditions and characters of the living beings, found at that period roaming at large in the once tropical swamps of the Wealden. The Wealden is almost if not wholly of fresh water origin; and a word or two on the formation of the deltas of great rivers, now going on in various parts of the world, will help us rightly to appreciate the character of this formation. It is seldom, some one remarks, that we “can catch a mountain in process of making,” and hence we have much difficulty in arriving at definite ideas concerning the times that the sedimentary rocks occupied in their deposit. But we _can catch_ deltas in the process of manufacture,—the deltas of the Nile, the Mississippi, the Amazon, and other huge rivers that pour down, not their “_golden streams_,” but their muddy accumulations, gathered from the mountain sides whose slopes they wash into the great ocean basin waiting to receive them. “All the rivers run into the sea, yet the sea is not full.”