ABSTRACT
The evidence provided by the nature of the rocks is more satisfactory, e.g. boulder clays indicating glacial climates, salt and gypsum deposits indicating aridity, coral limestones indicating warm seas, coal seams which, from the absence of annual rings of growth in the trees, have been interpreted as tropical forest vegetation. The most difficult single fact to explain in all the history of climate is the glaciation of tropical latitudes in Permo-Carboniferous times. The climate of Alaska would resemble that of Norway and there would be open water in winter perhaps as far as the mouth of the Mackenzie River. The air became drier, the climate more continental. New land masses began to interfere with the circulation of the oceans, new mountain chains excluded the moderating influence of the sea from the hearts of the continents, where deserts came into existence.
