Page:Popular Science Monthly Volume 76.djvu/483

Rh late Paleozoic era, about the line between Carboniferous and Permian, and that too in the regions around the Indian Ocean, where it is now tropical or subtropical in temperature.

Before we had entirely recovered from the shock of that discovery, it was found out that in China and Australia there was another glacial epoch, or epochs, near the beginning of Cambrian time. Now the geologist's spirit is so broken, that when the supposed discovery of glacial epochs in Silurian and Devonian time is announced, he hardly raises a dissenting voice, and appears to be resigned to the occurrence of glacial epochs at almost any time in the history of the earth. The theory of ancient climatic uniformity is definitely abandoned, and we must accept fluctuations of climate from the earliest geologic record all through the history of our planet. The old idea was delightfully simple, but too good to be true.

—Physical evidence as to ancient climatic conditions is limited to two classes—glacial deposits and ice-work, and sediments indicating desiccation, that is, saline and gypsiferous beds. These are both necessarily limited to continental areas, and tell us nothing of marine conditions. And as we go back in time they become more and more indefinite, so that there is much difference of opinion as to their value. The evidence of the recent Glacial epoch is positive enough to satisfy the most critical, but geologists are not yet united as to the glacial epochs in older periods of geologic history, because of the difficulty of determining whether the ice-masses were true sheets or whether they were mere local highland glaciers.

Also the sedimentary deposits indicating desiccation may have been merely local, and although they are positive as to prevalence of evaporation at that particular place, they can not tell positively of wide-spread dry climate, and certainly they do not indicate temperature.

Organic Evidence.—Fossil remains of animals or plants known to have lived in either warm or cold climates are more definite, and tell us equally well of land and water conditions, but they are authentic only when the fossils are animals or plants that have either lived on into our own time, or when the groups to which they belong have always had the same habits. This becomes more and more conjectural as we go back in geologic history, and have to deal not only with extinct species, but even with extinct genera, families and orders.

Extensive fossil beds of deciduous trees point to moist climates, and usually to temperate conditions. But deciduous trees extend back only to the middle of the Cretaceous, and beyond that time we have no positive criteria for temperate climate.

Cycads and palms are the best evidence as to tropical climates on