The third of the Eocene stages of the interior is the _Bridger_ of southern Wyoming and northeastern Utah, where it lies upon the Green River shales, but overlaps these shales both eastward and westward, extending out upon the Wasatch. The Bridger beds are largely made up of volcanic ash and dust deposited partly upon the land and partly in shallow or temporary lakes. The frequency with which the remains of fishes, crocodiles and fresh-water shells are found indicates deposition in water, and the large crystals of gypsum which are abundant in certain localities show that the water became salt, at least occasionally. From the immense mass of volcanic débris, it is evident that volcanic activity broke out at this time on a much greater scale than had been known in that region since the Cretaceous period. Two different horizons, or substages, are distinguishable in the Bridger, lower and upper, each of which has its distinct mammalian fauna, though the two are very closely allied. Their difference from the contemporary mammals of Europe is very great, hardly any genera being common to the two continents. So striking a difference indubitably points to a severance of the land-connection, a severance which, as was shown above, probably took place during the Wind River stage, for its effects would not be immediately apparent; time would be required for the operation of divergent evolution, the fauna of each continent developing along its own lines, to make itself so strongly felt. Had the connection never been renewed, North America, on the one hand, and Eurasia on the other, would to-day be utterly different from the zoölogical point of view, instead of containing, as they do, a great many identical or closely similar animals of all classes, a likeness due to subsequent migrations. The fourth and last of the stages referred to the Eocene is the _Uinta_, the geological position of which is the subject of much debate; almost as good reasons can be brought forward for placing it in the Oligocene as in the Eocene, so nearly is it on the boundary line between those two epochs. The Uinta is found in the Green River valley of northeastern Utah and northwestern Colorado, where it lies upon the upper Bridger and is the latest of the important Tertiary formations to be found in the plateau region west of the Rocky Mountains. It is probable that the separation of North America from the Old World still continued, for, as a whole, the Uinta fauna is totally different from that of the upper Eocene of Europe. There were, however, a few doubtful forms, which may prove to be the outposts of a renewed invasion.