INFLUENCE OF LIMESTONE UPON THE ATMOSPHERE 617 
continental seas furnish conditions congenial to the chief lime- 
secreting organisms. Preéminently is this true when approxi- 
mate base leveling attends the wide transgression of the sea, as 
it normally does, since the two conditions are cogenetic. At 
such times the waters are relatively free from land wash, and the 
extended shelves and the epicontinental seas have their greatest 
availabilities of depth. These are the conditions theoretically 
most favorable to limestone formation. These seem to be the 
conditions that actually prevailed at the epochs of great lime- 
stone deposition. At such periods great quantities of carbonic 
acid previously stored in the calcium bicarbonate of the ocean 
were set free and the atmosphere enriched in carbon dioxide. 
It was precisely during such periods of special enrichment that 
the land was most incompetent to impoverish the atmosphere, 
because it was then smallest and lowest. During the prevalence 
of these conditions it seems inevitable that the enrichment of 
the atmosphere should have become notable. A rude computa- 
tion may give some impression of the quantitative competency 
of a great deposition of limestone to set carbonic acid free. 
The limestones of the mid-Ordovician period may be taken as an 
example. According to the estimate of Dr. Tillo,* 17 per cent. 
of the land is covered by the Paleozoic series and 80 per cent. 
by the total sedimentary series. Taking no account of the loss 
by erosion, nor of the portions concealed by the ocean, and 
making the very conservative assumption that only one-fourth of 
the sedimentary area is underlain by this wide-spreading forma- 
tion, and the further excessively conservative assumption that 
the Ordovician limestones average only fifty feet exclusive of 
impurities, and that only a half equivalent of carbon dioxide 
was freed for every equivalent of calcium carbonate extracted 
from the sea water, we still find the amount of carbon dioxide 
set free to be sixty times the present carbon dioxide of the 
atmosphere. It is obvious that this amount could not be 
extracted from an ocean like the present one without concurrent 
supply, for it is about four times as great as the ocean’s avail- 
« BERGHAUS’ Physical Atlas. Introduction to Geological Maps. 
