oriCtIX and differentiation 559 



drifted by marine currents to the places where they now are. It is 

 of course well known that seeds and fruits of various kinds may be trans- 

 ported vast distances by ocean currents, and deep dredging by Agassi z 

 and others off the American coasts has disclosed the presence on the sea 

 bottom of plant debris, such as wood, branches, leaves, and seeds in 

 various stages of decay. In some instances these remains were fairly 

 abundant at a distance of 1,100 to 1,200 kilometers from shore, which 

 corresponds to about 10 degrees of latitude. 



But the plants in the Arctic regions fall within a quite different 

 categor}^, as they exhibit unmistakable evidence of having been deposited 

 in fresh water. Nathorst *^ has recently published a paper in which he 

 describes fully the depositional conditions under which the plants were 

 entombed. The deposits range in age from Devonian to Eocene, and 

 the plants occur often in shales overlying coal and are often underlain 

 by clays filled with rootlets and into which the plants, roots, and stems, 

 penetrate. There is little or no evidence of sorting by water, there 

 often being branches, leaves of various sizes, and fruits, evidently from 

 the same tree, all being in frequent association with Unios and other 

 fresh-water shells. The matter may be considered as settled, that what- 

 ever the explanation of the climatic conditions which permitted their 

 growth, full account must be taken of the fact that they undoubtedly 

 grew at or very near the places where they are now buried. 



Reversal of deep-sea circulation. — Some years ago Prof. T. C. Cham- 

 berlin ^* advanced a proposition, later strongly indorsed by Willis,*^ that 

 the present deep-seated equatorward circulation of cold oceanic waters is 

 abnormal, and that the more normal condition is a poleward movement 

 of warm, highly saline waters in the depths instead of at the surface. 

 This hypothesis was advanced to explain in perhaps major part the polar 

 geniality of climate that geologic and paleontologic evidence shows con- 

 clusively to have occurred again and again in the ages before the Pleis- 

 tocene. .Under present conditions the density of polar waters is due 

 primarily to cold, and this density may be increased by the forcing out 

 of salts from the superficial layers by freezing; the absence of large 

 rivers also contributes to the same end. If the polar cold could be 

 ameliorated, obviously these influences would be moderated. Similarly, 

 at the present time the waters in the equatorial regions are evaporated 



*3 A. 6. Nathorst : Fossil floras of the Arctic regions as evidence of geological climate. 

 Geol. Mag., new ser., vol. S, 1911, pp. 217-225. 



**T. C. Chamberlin : On a possible reversal of deep-sea circulation and its influencf 

 on geologic climates. Jour, Geology, vol. 14, 1906, pp. 363-872. 



4» Bailej' Willis : Science, new ser., vol, 31, 1910, p. 245. 



