THE PENNSYLVANIAN PERIOD. 603 



Lepidodendron and Sigillaria extended to that region, but Glossopteris, 

 singularly related to the glacial formations, seems to have been con- 

 temporaneous with them, while it did not reach Europe till the late 

 Permian and is not known to have reached North America at all. 

 With the present imperfect data for the southern hemisphere, it is 

 uncertain whether the northern coal flora, as a whole, ever extended 

 to that region, although some of its leading genera certainly did. 

 While recognizing the great uniformity of the Carboniferous vegeta- 

 tion of North America, Europe, and a portion of Asia, it is prudent 

 to hold opinion in reserve relative to the southern hemisphere. 



Climatic Implications of the Coal-plants. 



While the foregoing facts are fresh in mind, it may be best to 

 inquire what suggestions arise from the characters of the plants rela- 

 tive to the atmospheric conditions under which the Coal flora grew. 

 Two rather antagonistic views relative to these conditions have been 

 held. The older of these views regards the accumulation of the thick 

 deposits of coal as evidence of a very luxuriant growth of vegetation, 

 which in turn has been thought to imply a very warm, moist atmos- 

 phere, heavily charged with carbon dioxide. The abundance of tree- 

 ferns and of large equiseta, such as now are found only in tropical 

 countries, has been thought to strongly support this view. The occur- 

 rence of coal deposits and of the Coal flora in high latitudes has been 

 appealed to as proof of uniformity of climate. This view was formerly 

 held almost unanimously, and still preponderates. Its grounds seem 

 to be strong. The alternative view which has grown up in recent 

 years postulates less rapidity of growth, lees warmth and moisture, 

 and more diversity of atmospheric states, as well as a nearer approach 

 to the present atmospheric constitution, though it accepts a some- 

 what increased percentage of carbon dioxide, and assumes a milder 

 and more uniform climate than the present. The basis of this view 

 (held in different degrees and phases by different geologists) is found 

 in the following considerations. Great thicknesses of coal do not 

 necessarily imply rapid accumulation, any more than great thick- 

 nesses of limestone. Given favorable conditions of preservation, a 

 slow growth will produce the thicknesses realized. At present the 

 accumulation of peat, the nearest analogue of coal formation, indeed 

 the first step in coal formation, is most favored in cool climates, and 



