THE PERMIAN PERIOD. 675 



these areas is yet to be demonstrated, but one of the conceptions of their dy- 

 namics that seems to be warranted at present, seems also to be helpful in explain- 

 ing peculiarly localized glaciation. In this conception, mechanical and vapor- 

 ous energy are assigned a controlling value in determining the special configura- 

 tion of the air movements, while thermal energy is placed in the background 

 in a double sense. It is, of course, recognized that, at bottom, the chief agency 

 in atmospheric circulation is unequal heating. Unequal distribution of moisture 

 is also an original agency, since the superior kinetic energy of vapor, relative to 

 its weight, renders the air column of which it forms a part lighter than if formed 

 of dry air, other things being equal. This amounts to an appreciable agency. 

 While the amount of its effect is dependent on the absolute amount of vapor 

 present , its operative efficiency is dependent on the difference in its abundance 

 in the different parts of the atmospheric province, precisely as in the case of 

 heat. Starting from one or the other of these original sources, or from a com- 

 bination of the two, circulation is inaugurated ; but, when once under way, many 

 secondary factors of a mechanical, rather than thermal or vaporous nature, 

 are developed, and these appear to greatly influence the special configuration 

 which the air movements and even air pressures actually assume. Some of 

 these mechanical factors are closely derivative from the original thermal or 

 vaporous energies, as the exceptional violence of squalls, tornadoes, etc., while 

 some spring from the modifying effects of independent agencies, as the rota- 

 tion of the earth, the configuration of the surface, the distribution of land and 

 water, the ocean currents, etc. 



Now, it is conceived that in a region where the agencies last named are spe- 

 cially influential, and the influence of differential temperature is not particularly 

 strong, the mechanical factor and the vapor together may be dominant, and 

 as they are fixed geographically, the cyclonic area also becomes fixed, instead 

 of moving with the atmosphere, as do the more familiar cyclones. It is not 

 intended, however, to imply that even these are not measurably dependent on 

 derived mechanical energies. It is further conceived that in these fixed cyclonic 

 "lows," the moist air of the great central tract is mechanically forced to rise 

 at a relatively low temperature and the precipitation is hence favorable to glacial 

 accumulation. In a more purely thermal cyclone, the relatively high degree 

 of heat involved in immediately actuating the cyclonic movement is present to 

 antagonize glacial accumulation. This conception is extended to the moving 

 cyclones within the permanent cyclonic area, and perhaps even beyond. It is 

 to be understood that this is not established meteorological doctrine. These 

 are problems, like the rest of this complex of puzzles, yet awaiting final eluci- 

 dation. The special point of application here lies (1) in the fact that this con- 

 ception provides for precipitation with a minimum of heat by assigning mechan- 

 ical energy and vaporous levity as the dominant actuating agencies, and (2) in 

 that it gives a fixed location to the low-temperature precipitation that follows. 



Probable geographic features of the glacial stage. — If the land extensions 

 and connections in the Southern Hemisphere in the Permian period be made as 

 slight as biological data permit, they would probably at least consist of a con- 



