FOSSIL PLANTS AS AN AID TO GEOLOGY. 379 



the fossil plants, especially Nematophyton crassiim known from 

 the study of its internal structure, to prove the Middle Devonian 

 age of that part of the geological section. 



Certain well-defined species of fossil wood are characteristic 

 of particular horizons, as for example Cordaites Ouangojidianus 

 (Dn.) Gopp., which is confined to the Middle Erian (Devonian); 

 C. Halli (Dn.) Kn., and C. Newberryi (Dn.) Kn., are confined to 

 the Hamilton Group; Dadoxylon mi?iidatunt Dn., found only in 

 the middle coal-measures, etc. 



SUBSIDIARY USE OF FOSSIL PLANTS. 



Among the many relatively subsidiary problems connected 

 with the application of paleobotany to geology, the use of fossil 

 plants as tests of past climate occupies an important place. 

 Plants are unable to migrate like animals when the temperature 

 of their habitat becomes unfavorable, and they must either give 

 way, or adapt themselves gradually to the changed conditions 

 of environment. Hence, fossil plants have always been accorded 

 first place as indices of past climates. " They are," as Dr. Asa 

 Gray has said, "the thermometers of the ages, b}^ which climatic 

 extremes and climate in general through long periods are best 

 measured." ^ 



The wide geographical distribution and similarity of appear- 

 ance of Paleozoic plants, especially coal-measure plants, argues 

 beyond question a uniformity of climatic conditions. The 

 absence of rings of growth in the Carboniferous conifers shows, 

 as long ago pointed out by Witham, that the seasons, if such 

 they could have been called, were either absent or not abrupt, 

 and it is not until the Trias is reached that the clearly defined 

 rings of growth bear indisputable evidence of the existence of 

 seasons. 



" Heer, as a result of his examination of the Swiss Tertiary 

 plant-beds, is led to the interesting conclusion that in certain 

 cases it is possible to detect the regular recurrence of seasons 

 by the constant association in the same strata of fruits or leaves 



'The Nation, No. 742, September 18, 1879. 



