THE ALLEGED DESICCATION OF EAST AFRICA 15 
Abyssinia and Kilimanjaro this precipitation must have been 
enormous, and an enormous amount of water must have flowed 
eastwards towards the Indian Ocean. 
Prof. Gregory also produces arguments with regard to the 
effect of the variation of lines of atmospheric pressure which 
would be caused by the greater elevation of a large area, and 
which he claims would greatly increase the rainfall and widen 
its distribution. I am, however, inclined to doubt whether 
the information as to the position of the isobaric lines, i.e. 
lines of equal barometric pressure, is sufficiently accurate to 
build upon, although this would of course not necessarily 
affect the principle of the argument. 
Several other explanations have been adduced by geologists 
to account for periodic variations in climate, and variations 
in rainfall would follow. One of these, which has from time 
to time attracted considerable attention, is that the main 
contributory cause is the variation in the carbonic acid contents 
of the atmosphere. There is little doubt that in earlier 
geological times the proportion of free carbonic acid gas was 
much greater in the atmosphere than at present ; all the great 
coal deposits of the world are built up of carbon from the 
atmosphere, and it is alleged that the enormous deposits of 
limestone in the Earth’s crust, which contain many times more 
carbon than all the coal deposits put together, although 
primarily they obtained their carbon from the sea, absorbed 
a great portion from the air. The evidence, as far as it goes, 
appears to prove that the oceans are the greatest governors 
of the proportions of carbonic acid gas in the atmosphere, and 
it is calculated that an increase of 0 06 of carbonic acid gas 
in the atmosphere would cause a rise in the average temperature 
in the Polar Regions of some 14° Fahr., the idea being that 
a reduction in the carbonic acid gas contents cools the climate 
and an increase causes the reverse. 
Now, extensive volcanic action is known to produce vast 
amounts of carbonic acid gas, but the great volcanic activity of 
Pleistocene times does not seem to fit in altogether with the 
greater extent of the Kenya glaciers about that period. I 
believe, however, that extensive volcanic activity is usually 
attended by temporary torrential rainfall, partly due to the 
