474 



Transactions. — Geology. 



pressure for the whole earth's surface at sea-level of 14-6251b. 

 to the square inch. Fig. 1 : — 



Pressure of Atmosphere. 



Total for Earth. 

 Av. 14-6251b. 



Second, let it be supposed that the same area of land existed 

 at the beginning of the Pleistocene period as at present, but 

 that the contrasts were wider, and that the average height of 

 the land was 10,200ft. The barometric pressure, assuming 

 that the same aerial envelope surrounded the earth, would then 

 have been 21in., which is equal to 101b. to the square inch. 

 This would give for the remaining three-fourths represented 

 by the water-surface a pressure of 16-61b. to the square inch, 

 the height of the barometer over the same area being 33-33in. 

 Fig. 2 illustrates this by the graphic method : — 



As a third example, let it be assumed that half the earth's 

 surface was occupied by the land at the opening of the Pleis- 

 tocene period, the average height being 5,620ft. The baro- 

 metric pressure over the laud would average 25in., correspond- 

 ing to 12 -lib. to the square inch. This would give a surface 

 pressure over the other portion of the earth amounting to 

 17 -151b. to the square inch, the barometer showing a height of 

 35-43in. Fig. 3 illustrates this by the graphic method : — 



Pressure of Atmosphere. 



Water, 17-151b. bar. pressure to ! Land, 12-llb. bar. pressure 



to 



square inch. 



square inch. 



We know the physical conditions resulting from the atmo- 

 spheric pressure as shown in fig. 1, as these conditions exist 

 now ; but what would probably be the characteristics of the 

 animal and vegetable life assuming that the earth presented 

 either of the conditions illustrated by figs. 2 and 3 ? Under 

 fig. 1 the temperature contrasts may be assumed to vary 



