ANCIENT CLIMATES 479 



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 geol- 

 ogist'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. 



Criteria of Ancient Climates 



Physical Criteria. — Physical evidence as to ancient climatic condi- 

 tions is limited to two classes — glacial deposits and ice-work, and sedi- 

 ments 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 diffi- 

 culty 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 evapora- 

 tion 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 



