246 POPULAR SCIENCE MONTHLY. 



the gases projected by the sun beyond its gravitative control, in inter- 

 stellar spaces, in meteorites and in the terrestrial mass. The first three 

 possible sources are too indefinite both in amount and in distribution 

 through the ages to be of any present value to the hypothesis, but the 

 last is important. Crystalline rocks of the superficial crust of the earth 

 are shown by anal3^sis to contain four and one-half times their own 

 volume of gases, of which carbon dioxide, CO2, and monoxide, CO, 

 form a large percentage. During volcanic eruptions gases and vapors 

 are ejected in indefinitely large volumes. What part of these was 

 once of the atmosphere and is returned to it after an underground 

 journey we do not know, but it is believed that a large part may come 

 from the interior. The escape of these internal gases may also occur 

 in some degree continuously by diffusion, and in influential amounts 

 during episodes of mountain growth, when rock masses are strained 

 and riven and upraised. We shall see presently that in the wasting of 

 a mountain range there is serious consumption of carbon dioxide, 

 which in greater or less degree temporarily affects the gain. The 

 Pleistocene glaciation is attributed to a very notable offset of this char- 

 acter, but the exceptional nature of that event as compared with the 

 relatively frequent episodes of mountain growth indicates that the gain 

 of carbon dioxide has commonly equaled or exceeded the resulting tem- 

 porary loss. 



Among the cycles of combination and release, through which car- 

 bon dioxide runs, there are two which are both important, though 

 not equally so. The first and less important is the cycle of organic 

 change, involving plant and animal tissues. When grass grows, carbon 

 dioxide is taken from the air. Grass becomes beef, and beef, through, 

 various changes', is resolved into new compounds, yielding back the 

 carbon dioxide to the air. In its brief phases this cycle has no import 

 for the hypothesis, but there are occasions where it is prolonged, as in 

 the accumulations of vegetal substances fossilized as coal. The total 

 amount of carbon dioxide thus abstracted, and now withheld, is very 

 large, and is believed to have been an important factor in promoting 

 at least one instance of glaciation, that which followed closely upon the 

 Coal Measure period. 



The more important cycle of combination and release involves the 

 decomposition of rocks by weathering, the solution of certain products 

 and their transportation to the sea, and the reactions through physical, 

 chemical and organic agencies, by which carbon dioxide is either per- 

 manently locked up in limestones or is returned to the air. 



For the purposes of this statement, the common minerals of rocks 

 may be classified as silicates and carbonates, that is as compounds with 

 silicic acid and compounds with carbonic acid. The former may be 

 typified by the familiar minerals of granite, the latter by limestone. 



