Still further back, in the Jurassic period, we have proofs of a mild climate in East Siberia and at Andö in Norway just within the Arctic Circle, in numerous plant remains, and also remains of great reptiles allied to those found in the same strata in all parts of the world. Similar phenomena occur in the still earlier Triassic period; but we will pass on to the much more remote Carboniferous period, during which most of the great coal-beds of the world were formed from a luxuriant vegetation, consisting mostly of ferns, giant horse-tails, and primitive conifers. The luxuriance of these plants, which are often found beautifully preserved and in immense quantities, is supposed to indicate an atmosphere in which carbonic acid gas was much more abundant than now; and this is rendered probable by the small number and low type of terrestrial animals, consisting of a few insects and amphibia. But the interesting point is, that true coal-beds, with similar fossils to those of our own coal-measures, are found at Spitzbergen and at Bear Island in East Siberia, both far within the Arctic Circle, again indicating a great uniformity of climate, and probably a denser and more vapour-laden atmosphere, which would act as a blanket over the earth and preserve the heat brought to the Arctic seas by the ocean currents from the warmer regions. The still earlier Silurian rocks are also found abundantly in the Arctic regions, but their fossils are entirely of marine animals. Yet they show the same phenomena as regards climate, since the corals and cephalopodous mollusca found in the Arctic beds closely resemble those of all other parts of the earth.[16]