During the last few years much study has been given to “palæoclimatology,” but such a study is extremely difficult. Only a very small fraction of the total surface of the earth can be geologically examined, and of that fraction a still smaller proportion has up to the present been studied in detail. There has been a great tendency to study intently a small region and then to generalize. The method of study which has to be employed is extremely dangerous. A geological horizon is determined by the fossils it contains. Wherever fossils of a certain type are found the strata are given the same label. Isolated patches correlated by their fossils are found in different parts of the world, and it is frequently assumed not only that these rocks were laid down at the same time, but that the conditions which they indicate existed over the whole of the earth’s surface simultaneously. Thus geologists tell us that the climate of the Carboniferous Age was warm and damp; of the Devonian Age cool and dry; of the Eocene Age very warm; of the Ice Age very cold. But has the geologist given sufficient attention to the climatic zones during the various geological climates? It is true that the geologist has definitely expressed the view that in certain ages climatic zones did not exist; but from a meteorological point of view it is difficult to see how the climate could have been even approximately the same in all parts of the world if solar radiation determined in the past as in the present the temperature of the surface of the earth. The climatic zones of the various geological periods will need much closer study in the future; the data hardly exist at present, and the great area covered by the ocean will always make the study difficult and the conclusions doubtful. Admitting, for the sake of argument only, large changes in average conditions, but with zonal variations of the same order of magnitude as those existing to-day, the slow changes from period to period will cause any given climatic state to travel slowly over the surface of the earth, and this will so complicate the problem as to make it doubtful whether any conclusions can be reached so long as the same criteria are used to determine both the geological epoch and the climatic conditions.