662 GEOLOGY, 



by the ocean if it were as nearly saturated as was possible, and that without 

 adversely affecting the animal life. It cannot of course be affirmed that this 

 was the case; indeed the probabilities are against it for reasons that will soon 

 appear. But, to make the most of this resource, let it be supposed that, at the 

 opening of the Mississippian period, the ocean contained, by virtue of its 

 carbonates, 60 to 80 equivalents of the present atmospheric carbon dioxide. 

 Even this would fall far short of furnishing all the carbon dioxide locked up 

 in the limestones of the period. It would, however, make a large contribution 

 to the requisite amount, and thereby help appreciably to solve the problem, 

 'provided it be assumed that the carbonates were removed from the ocean in the 

 course of the period by consumption in limestone formation. 



Could this have taken place? The conditions for the extraction of calcium 

 carbonate by lime-secreting organisms were especially favorable. The map 

 of the period (Fig. 228) indicates that, aside from the continental shelves, about 

 5,000,000 square miles of the continental area of North America were, by esti- 

 mate, submerged, or more than half the present land area of North America. 

 Great encroachments are known to have taken place on other continents, and a 

 similar estimate of submergence of one half the present land area is probably 

 in the neighborhood of the truth. If the submerged continental area for the 

 globe be placed at 20,000,000 square miles, in addition to the 10,000,000 of the 

 platform now submerged, it gives a vast tract of water peculiarly suited to be 

 the habitat of lime-secreting life. When it is borne in mind that nearly the whole 

 of the secretion of lime takes place in the upper few hundred feet of the sea, 

 (1) through the agency of pelagic life whose habitat would be 10% greater than 

 now, and (2) by epicontinental life whose habitat would be 200% greater than 

 now, the potentiality of the geographic change may be realized. It is further 

 to be noted that the warm temperature of the times not only favored an abun- 

 dance of lime-secreting life, but favored the process of lime-secretion itself, for 

 it is observed that the animals of the warm seas secrete more lime, on the aver- 

 age, than the same types in cold seas. 



On the other hand, it is to be noted that the supply of carbonates from 

 the land area was reduced in proportion as the sea was extended, and the 

 land that remained unsubmerged was probably low, and had previously 

 been much leached during the base-leveling processes. Hence the land waters 

 penetrated the earth but feebly, from lack of head, and from hindrance by 

 the deep mantle of residuary earth that presumably covered the low lands, 

 and the carbonation and leaching of the lands was therefore relatively feeble. 

 The seas were therefore much less rapidly supplied with calcium carbonate than 

 in periods when the lands were broader and higher, and the penetration of the 

 surface-waters deeper and more active. The period therefore presented con- 

 ditions favorable for the extraction of lime from the ocean waters, and unfavor- 

 able for renewing the supply,, Under these conditions, unless there be offsetting 

 agencies not now recognized, it may be assumed that the carbonates of the sea 

 were gradually reduced. 



While the chemical relations involved are very complex and have not been 



