ROBBiN^s^"^] CLIMATE AND EVIDENCE OF CLIMATIC CHANGES 61 



ice spread over large portions of Europe, northern Asia, and North 

 America, White and Know! ton say that paleontologically we are 

 still in the Glacial epoch, a period of radical changes. Glacial evi- 

 dence in the Rocky Mountains also connects the glaciation of the West 

 with the present. 



The geologic evidence of a change of climate in the immediate 

 vicinity of the Rito de los Frijoles is very meager or entirely absent. 

 However, when southwestern United States is considered as a whole, 

 there is conclusive evidence of geologically recent widespread change 

 in climate, probably continuing to the present time, which must have 

 affected the area under discussion. 



The existence of surface water in its various forms (i. e., as water, 

 ice, and snow) depends not entirely on the amount of piecipitation 

 but rather on the relation of precipitation to dissipation. Dissipa- 

 tion may take place by ''run-off" through streams, by seepage into 

 soil and rocks, and by evaporation. Evaporation in ^he Southwest 

 at the present time is a very important factor, vitally affecting both 

 plant and animal life and enormously affecting the culture of the 

 region. It bears a direct relation to the force, continuity, direction, 

 temperature, and humidity of air currents. Hence any influence 

 that would increase or decrease either the temperature or the humidity 

 of atmospheric currents, or change their velocity, continuity, or 

 direction, would manifest itself in the increased or decreased comfort, 

 happiness, and prosperity of the human inhabitants. Precipitation 

 is dependent largely on the distribution of large bodies of land and 

 water and the relation of moisture-laden air currents to such elements 

 of topography as mountains and plains. Hence the causes of regional 

 climates and climatic changes form always a liighly complex problem, 

 and though one may determine in many cases from biologic and geolo- 

 gic evidence that changes have occurred in the relation of precipitation 

 to dissipation, it is not so easy to determine whether such changes 

 have affected the one, or the other, or both, or to ascertain the causes 

 of the changes. It may be said that probably in most cases a change 

 in either precipitation or dissipation would react to some extent on 

 the other, so that if there has been progressive desiccation in the 

 Southwest within recent times, it has probably been the result of 

 both decreased precipitation and increased evaporation. The com- 

 plexity of the problem and the local capriciousness of rainfall, wind, 

 etc., make it difficult or impossible with known methods and few 

 stations to record all the constantly changing factors in such manner 

 as to accurately and adequately plat the curve of the ratio of pre- 

 cipitation to evaporation in a given region. As the existence and 

 extent of land-locked lakes and glaciers are the direct results of this 

 ratio after gixnng proper value to all factors, the records of their fluc- 

 tuations, if complete, would furnish the best possible evidence as to 



