Geography of Ohio 211 



and on the lowered lands plant growth became more luxuriant. 

 As the base-leveling proceeded, the limestones and other rock 

 horizons were disintegrated ; the amount of carbon dioxide in the 

 ocean, as well as in the atmosphere, was increased. Extensive 

 lime deposits were again made and luxuriant plant life spread over 

 the base-leveled continents. With the formation of limestone, 

 the quantity of carbonates was much reduced in the oceans ; and 

 the unusual consumption of carbon dioxide by vegetation tended 

 to deplete the atmospheric content of this gas. With a reduction 

 also of the water-vapor of the atmosphere, heat was quickly 

 radiated from the lands. The mean annual temperature of the 

 earth was much reduced; in these parts where ocean currents, 

 winds, and land areas maintained the correct relationship, pre- 

 cipitation in the form of snow followed. Succeeding the base- 

 leveling, deep-seated crustal movements, consequent probably 

 on the over-loading of the ocean basins and lightening of the con- 

 tinents, tended to uplift the land and possibly also to elevate some 

 submerged tracts, either forming new lands, or at least changing 

 the course of ocean currents. This matter of elevation of land 

 areas is a contributory factor, probabl}', in deciding the form of 

 precipitation ; furthermore, the increased elevation of the lands 

 facilitated the carbonation of rocks, thus tending to further deplete 

 the atmosphere's carbon dioxide. Only in the parts of continents 

 where the prevailing winds and sufficient altitude, or the correct 

 latitude, combined to induce snow, did ice caps develop. Snow- 

 fields might result, therefore, either in the tropical or in higher 

 latitudes. 



It is well understood that the direction of ocean currents is partly 

 controlled by the location of land areas. If, for example. North 

 and South America were not connected, and the northern part of 

 the equatorial drift current were to continue directly west, instead 

 of being diverted northward in the ''Gulf Stream," as it now is, 

 great changes in the climate of Europe would result ; there might be 

 greater snowfall in western Europe. On the basis, then, of a 

 cyclic variation in the atmosphere's content of carbon dioxide, 

 following base-leveling periods, and their possible coincidence 

 with such changes in the outlines of continents as might alter the 

 courses of some ocean currents, we have, at least, a working hypo- 

 thesis for the several glaciations and their geographical distribu- 

 tion during geologic time. 



