316 A. P. Colemcm — Paleobotany and the 



aridity during several periods of the earth's history 

 should make little impression on a paleobotanist. 



To the physical geologist this bias towards one type of 

 evidence, mainly biological, seems unjustified. The phys- 

 ical proofs of comparatively short but tremendously im- 

 portant times of climatic stress seem overwhelming and 

 completely cut the ground from under the Nebular Hy- 

 pothesis as advocated by Manson. 



It seems to the present writer that Dr. Knowlton has 

 entirely underestimated the importance of ancient ex- 

 tremes of climate, and it is proposed to recall briefly the 

 salient points which oppose the Mansonian theory. For 

 this purpose one may consider especially three lines of 

 evidence, those indicating desert conditions, those prov- 

 ing seasonal variations and, above all, the prevalence of 

 ice ages at various times in the past. 



Neither times of drought nor changes of seasons nor 

 extensive sheets of ice on low ground are compatible with 

 a warm earth covered with impenetrable clouds and thus 

 screened from the influence of the sun. 



Evidences of aridity. 



Dr. Knowlton objects to the inference that red rocks 

 imply desert conditions, and in this he is justified to the 

 extent that red rocks may be formed from red materials 

 under all climates ; but to deny that there were deserts in. 

 the Triassic or Permian or Silurian is to ignore all 

 the known evidence ; and to dispute the formation of salt 

 and gypsum beds by evaporation in times of dry heat 

 without suggesting any other mode of forming such de- 

 posits is surely unwarranted. When one adds to this evi- 

 dence of evaporation on a large scale the desert sand- 

 stones containing wind-carved dreikanters, the playa de- 

 posits and the sun-cracked surfaces of mud preserved at 

 various ages reaching back to the pre-Cambrian, one finds 

 it difficult to believe in a warmly humid world enveloped 

 in rain clouds that never parted to let in the sun until the 

 Pleistocene. 



Evidences of seasonal changes. 



Dr. Knowlton 's references to the annual rings of forest 

 trees show that at least some seasonal change from warm 

 to cold or from wet to dry occurred in the Pennsylvanian, 

 the early Permian, the Jurassic and the Cretaceous. He 

 does not explain how this can be reconciled with the uni- 



