CLIMATOLOGY OF THE LATE PALEOZOIC 245 



than would be demanded for floating detached portions of mountain or plateau 

 glaciers entering the sea in their zone of melting." 



[The abundant evidence of Permian glaciation, etc., make it] "quite as reason- 

 able to suppose that ice formed on the fresh waters of the Carboniferous." [Also, 

 he says of a bed near the top of the Roxbury conglomerate:] "This presumably 

 tillite bed is possibly of Permian age, but its association with the underlying con- 

 glomerates and similar thick, water-worn conglomerates of known Carboniferous 

 (Allegheny) age in the Narragansett area points to the correctness of Shaler's 

 theory of the glacial origin of the conglomerates as a whole." 



The western instance is still an isolated one and may hardly yet be con- 

 sidered as definite proof of such low temperatures as are suggested by the 

 authors cited. 



Other evidence of a decided lowering of the temperature in Permo- 

 Carboniferous time is furnished by the change in life. David White, in the 

 paper quoted (page 238), has noted the "rapid decrease, approaching 

 extinction, of the colossal lycopods (Lepidodendrise), and the rapid develop- 

 ment of the group of gigantic tree ferns, such as Psaronius," with minor 

 changes; all of which he regards as insufificient to indicate a great climatic 

 change, but are clearly the beginning steps of what came later. Similarly 

 slight but progressive changes are noted in Monongahela time, which, 

 however, David White holds are still so slight that they indicate a moist 

 and warm climate with absence of killing frosts or long-continued periods 

 of aridity. In Dunkard time, however, the change was rapid and resulted 

 in the formation of distinct climatic provinces. White does not believe 

 that the eastern portion of the United States was extensively affected because 

 of the continuation of the coal during the Permo-Carboniferous. 



Aside from the change in the composition of the flora, the physiological 

 adaptations have been used in an interpretation of climatic conditions. 

 Xerophilous adaptations have been found in many plants of the late Paleo- 

 zoic, and these have been interpreted as indicating a decided increase in 

 aridity, but it is evident from the work of the botanists that xeromorphy in 

 plants is attributable to more than one cause and is still incapable of exact 

 interpretation. The occurrence of such structures in bog plants is well 

 known, but its meaning is still in dispute. The matter has been reviewed 

 by Clements,^ and without entering at length into the matter it may be 

 stated that it has been suggested that the effect of decomposition in stagnant 

 bogs and swamps is to produce both acidity and toxic products. It is 

 assumed by some that the presence of one or both of these acts as a deterrent 

 to root growth or functional activity and the xerophytic structure is a 

 response to the inability of the plants to obtain an adequate supply of 

 physiologically wholesome water. The literature of this matter may be 

 followed from Clements's summary and from Dachnowski.^ 



> Clements, F. E., Plant Succession: An Analysis of the Development of Vegetation, 



Carnegie Inst. Wash. Pub. No. 242, p. 90, 1916. 

 ^ Dachnowski, A., The Problem of Xeromorphy in the Vegetation of the Carboniferous 



Period, Amer. Jour. Sci., 4th series, vol. 32, p. 33, 191 1. 



