STAGES OF THE EARTH'S HISTORY 429 



The assumption back of this seems to be that heat always promotes 

 the expulsion of gases; if so, the separation of the gases from the 

 rock should have been most complete in the white-hot primitive 

 globe. This conception has been widely entertained, and the 

 reverse conception that the cooled rocks re-absorb the atmospheric 

 constituents is expressed in the once prevalent view that the former 

 atmosphere and hydrosphere of the moon have been absorbed into 

 that body, and the familiar prophecies of a similar doom for the 

 atmosphere and hydrosphere of the earth. 



Adverse physical evidence. So great an atmosphere with so 

 much carbon dioxide and water-vapor should have given the earth 

 a warm and equable climate. Such climates indeed seem to have 

 prevailed at certain times during the earlier parts of the earth's 

 history, as also during the later; but the studies of the past two 

 decades have shown that there was extensive glaciation on the very 

 borders of the tropics, as early as the close of the Paleozoic, and 

 that there was glaciation in northwestern Europe, in China in Lat. 

 31, in Australia, and perhaps in South Africa, as far back as the 

 very beginning of the Paleozoic. Less striking, but perhaps not less 

 significant, is the occurrence of extensive beds of salt and gypsum 

 in the early Paleozoic, in rather high latitudes. These deposits 

 seem to imply severe and protracted aridity, not readily recon- 

 cilable with an enormous, equalizing atmosphere. 



There seem to have been, in Paleozoic times, much the same 

 alternations of very uniform with very diversified climates that 

 marked the Mesozoic and Cenozoic eras. In other words, changes 

 of climate seem to have been much the same in early as in late 

 geologic time. 



Adverse organic evidence. The more the early life is compared 

 with modern life, the more nearly does it appear to imply the same 

 atmospheric conditions, and the more insecure does any view be- 

 come which postulates conditions profoundly different from those 

 of to-day. The air-breathing animals which lived early in the 

 Paleozoic era, and the xerophytic (indicating aridity) organs of 

 plants that lived in the later part of that era, seem irreconcilable 

 with a vast, hot, vaporous atmosphere, overcharged with carbon 

 dioxide and water-vapor. 



