ORIGIN AND DIFFERENTIATION 561 



It might to a certain extent facilitate the equatorward migration of 

 floras that originated within polar circles, but would it not soon erect a 

 cold, perhaps impassable barrier in middle latitudes? That is, if th(^ 

 waters were warmed in the equatorial region and sank to the bottom to 

 emerge in polar areas, the}^ would begin to flow equatorward, gradually 

 losing heat until somewhere in middle latitudes, and before they again 

 came under the warming influences of the equatorial region there would 

 be a point of maximum cold. Thus as a bar to the free interchange of 

 floras between opposite high latitudes, such as undoubtedly occurred,, 

 there would be erected three barriers, two cold areas in opposite middle 

 latitudes, separated by a warm area in the equatorial region. 



The ascertained facts of plant distribution will not tolerate any such 

 interpretation, and therefore it is much to be doubted that such reversal 

 did or could have taken place. 



There is also another point that may be urged against this theory of 

 reversed circulation. The terms of this hypothesis call for a heating 

 area in the equatorial region, and this in turn implies a solar control of 

 terrestrial temperatures similar to that now obtaining. As already 

 pointed out again and again, this could only result in a zonal distribu- 

 tion of temperatures which would seemingly nullify the ameliorating- 

 effect that might be brought about by warm waters rising within the- 

 polar circles. 



Volcanic dust as a climatic factor. — Benjamin Franklin was one of 

 the first, perhaps the first, to call attention to the possibility of volcanic 

 dust as a factor in the production of climatic changes. It seems that 

 during the summer of 1783 there was a constant fog over all of Europe 

 and most of Korth America, and the winter of 1783-84 was unusually 

 severe. He noted that Hecla, in Iceland, and adjacent volcanoes were 

 in active eruption, and suggested this as a possible cause of the cloudiness- 

 and hence of the lowered temperature. 



It is only in recent years, however, that accurate observational data 

 have been available on which to base a dependable conclusion as to the 

 possible effect of volcanic dust particles in the upper air in shutting off 

 the sun's rays. W. J. Humphreys *^ especially has published several 

 papers on this subject. It is now possible to measure the size of volcanic 

 dust particles with reasonable accuracy and to determine their time of 

 fall. He states that in middle latitudes the temperature of the atmos- 

 phere becomes substantially constant at an average elevation of about 11 



*'' W. J. Humphreys : Volcanic dust as a factor in the production of climatic changes. 

 Washington Acad. Sci. Jour., vol. 3, 1913, pp. 365-371. 



