Page:The Journal of geology (IA journalofgeology21894univers).pdf/395

the fossil plants, especially Nematophyton crassum 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 Ouangondianus (Dn.) Göpp., which is confined to the Middle Erian (Devonian); C. Halli (Dn.) Kn., and C. Newberryi (Dn.) Kn., are confined to the Hamilton Group; Dadoxylon annulatum Dn., found only in the middle coal-measures, etc.

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, by which climatic extremes and climate in general through long periods are best measured."^[1]

The wide geographical distribution and similarity of appearance 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 examinations 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