322 PHYSIOGRAPHY 



greater range of temperature, than has been common throughout 

 the larger part of the earth's history. 



Repeated changes in humidity seem to be as clearly indicated 

 as changes in temperature. Regions which have moist climates 

 now (e. g., New^York and Ohio) have been arid at times in the past, 

 and regions which are arid now (e. g., Arizona) have enjoyed moist 

 climates at times in the past. The former aridity in* the first case 

 is shown by salt and gypsum deposits, and the humidity in the 

 latter case is known by conclusive evidence of luxuriant plant life 

 in regions which are now nearly desert. There are, for example, 

 extensive "petrified forests" in Arizona (Fig. 3, PL XLVII, p. 273), 

 where many petrified logs are found, buried in the clay and sand 

 which were deposited here after the trees grew. 



From what is now known of the climates of the past, it seems 

 clear that causes have long been in operation which bring about 

 variations both in temperature and humidity. These causes have 

 been thought to be (1) geographic, and due to the changes in the 

 relations of land and water, or to changes in the topography of the 

 land; (2) astronomic, due to changes in the shape of the earth's 

 orbit, etc.; and (3) atmospheric, due to changes in the constitution 

 of the atmosphere. Still other causes have been conjectured. As 

 the facts concerning these changes are studied, they seem to be 

 pointing to the third of these lines of explanation as the most 

 probable. It cannot be said, however, that final conclusions have 

 been reached. 



