664 GEOLOGY. 



The remaining source of carbon dioxide was found in volcanic emanations 

 from the interior of the earth, and in the unknown supplies that came from cosmic 

 sources. 



All the foregoing sources combined may not improbably have been compe- 

 tent to furnish not only the carbon dioxide required for the formation of the 

 limestone, but also for such other carbonate and carbonaceous deposits as took 

 place. The feature of most note is the function assigned to the carbonates 

 and carbon dioxide of the ocean, which, by being depleted during stages of great 

 sea-transgression, meet the exceptional demands of the great limestone for- 

 mations that characterize such periods, while at the same time, as the result of 

 this, they leave the atmospheric-oceanic supply of carbon dioxide in a critical 

 state of reduction. 



The part played by coal-formation. — If the exceptional carbonic demands 

 of the great Subcarboniferous limestone formation be met thus, in part, by deple- 

 ting the ocean of its store of carbonates and carbon dioxide, the close of the 

 period would have found this resource exhausted, and unavailable for meeting 

 any exceptional demands during the following period; but it has been noted 

 that there was an extensive withdrawal of the sea, in America at least, between 

 the Subcarboniferous and Carboniferous periods. Much erosion and decom- 

 position must have taken place to give the cherts and quartzose material of the 

 Millstone grit and equivalent formations. So great thicknesses of chert as occur 

 at many points imply the solution of much limestone, and this in turn implies 

 large supplies of calcium carbonate carried to the ocean during the erosion inter- 

 val; while the mere fact that the ocean was extensively withdrawn, and lime- 

 secreting life restricted, implies a reduction of the drafts made upon its calcium 

 carbonate for limestone formation. To make this reasoning altogether firm, the 

 conditions in North America must, of course, have been representative of the 

 world at large. In the aggregate, this is fairly warranted, but not conclusively 

 shown, by present evidence. It may, however, be assumed tentatively that 

 when the main deposits of coal and limestone began in the Carboniferous period, 

 the ocean had become, in some measure, re-enriched in carbonates and loose 

 carbon dioxide. During that period there were considerable limestone deposits, 

 though these were not equal to those of the preceding period; and there were 

 also large accumulations of coal and other carbonaceous materials which made 

 heavy drafts on the supplies of carbon dioxide. Here again, the data for a 

 numerical estimate are very imperfect. The productive coal fields of this age 

 in North America are estimated to cover 265,000 square miles. North America 

 embraces about one seventh of the continental surface of the globe, but has rela- 

 tively more coal of this age than other continents. For a rough approximation, 

 the whole may be put at two or three times that of North America. If the impure 

 coals and other carbonaceous matter be included, an average thickness of 25 

 to 40 feet for the whole area may be assumed, without pretension to accuracy. 

 The carbon dioxide of the present atmosphere, if reduced to pure coal, would 

 give a layer about 1 foot thick over an area of 500,000 square miles, or about 

 8 inches thick over an area of 800,000 square miles. If there were no other 



