We cannot certainly affirm anything respecting the origin and migrations of these floras, but there are some probabilities which deserve attention. The ferns and cycads of the so-called Lower Cretaceous of Greenland are nothing but a continuation of the previous Jurassic flora. Now this was established at an equally early date in the Queen Charlotte Islands,[104] and still earlier in Virginia.[105] The presumption is, therefore, that it came from the south. It has indeed the facies of a southern hemisphere and insular flora; and probably spread itself northward as far as Greenland at a time when the American land was long, narrow, and warm, and when the ocean currents were carrying tepid water far toward the arctic regions. The flora which succeeds this in the sections at Atané and Patoot has no special affinities with the southern hemisphere, and is of a warm temperate and continental character. It is very similar in its general aspect to that of the Dakota group farther to the south, and this is probably Middle Cretaceous. This flora must have originated either somewhere in temperate America, or within the arctic circle, and it must have replaced the older one by virtue of increasing subsidence and gradual change of climate. It must therefore have been connected with the depression of the land which took place in the course of the Cretaceous. During this movement it spread over all Western America, and as the land again arose from the sea of the Niobrara chalk, it assumed an aspect more suited to a cool climate, or moved southward, and finally abandoned the Arctic regions, perhaps continuing to exist on the Pacific coast, and in sheltered places in the north, till the warm inland seas of the Upper Cretaceous had given place to the wide plains and land-locked brackish seas or fresh-water lakes of the Laramie period (Eocene). Thus the true Upper Cretaceous marks in the interior a cooler period intervening between the Middle Cretaceous and the Lower Eocene floras of Greenland. [104] Reports Geological Survey of Canada. [105] Fontaine has well described the Mesozoic flora of Virginia, _American Journal of Science_, January, 1879. This latter established itself in Greenland, and probably all around the Arctic circle, in the mild period of the earliest Eocene, and as the climate of the northern hemisphere became gradually reduced from that time till the end of the Pliocene, it marched on over both continents to the southward, chased behind by the modern arctic flora, and eventually by the frost and snow of the Glacial age. This history may admit of correction in details; but, so far as present knowledge extends, it is in the main not far from the truth.