INTRODUCTION. 



Climate as an element of physical environment is so well recognized that there is no 

 need to demonstrate its importance. By common consent it is held to be a primary 

 factor not onlj^ in the life of plants, animals, and man as they exist to-day, but in their 

 entire evolution. Moreover, among the main elements of physical environment it alone is 

 subject to pronounced changes in comparatively brief periods. The form of the lands, 

 the location of mountains, and the composition of the atmosphere are doubtless all subject 

 to great changes, but these are too slow to have much effect upon a single generation of 

 living beings or even upon all the generations that have existed during the period since 

 man emerged from barbarism. Small climatic changes, however, such as those from one 

 year to the next, or from one decade to another, are constantly in progress, and their far- 

 reaching results are a matter of every-day experience. Moreover, it is quite possible that 

 larger changes have taken place during the past 2,000 or 3,000 years, and if this is the 

 case their effects must have been correspondingly important. The investigation of this 

 possibihty is the purpose of this volume. 



The study of changes of climate natural^ divides itself into two parts, relating to the 

 present and to the past. The facts as to the present are being rapidly gathered by the 

 excellent work of the various National Weather Bureaus of the world. The facts as to 

 the remote past are being studied minutely bj^ geologists so far as they relate to geological 

 times. Comparatively little, however, is known as to the state of affau's during the period 

 covered by history and man's later development. Yet a knowledge of this period is 

 essential. In the first place, it is only by accurate knowledge of past variations that we 

 may hope to ascertain the causes of present variations, and thus to predict those which 

 will occur in the future. Geological evidence of course tells us much about the past, but 

 it pertains largely to periods too remote to be of great present importance, and its phe- 

 nomena can not be dated with accuracy in terms of years. Hence something else is needed 

 to fill the gap between such geological phenomena as the glacial period and our modern 

 climatic records covering scarcely a century. In the second place, a mathematical investi- 

 gation of the chief effects of present climatic conditions may do much to show how far 

 human habits, customs, physiological traits, and mental character are influenced by 

 physical environment, but it is impossible to determine the exact effect of present con- 

 ditions until we know how long those conditions have lasted and how the environment of 

 the past, especially during the last 2,000 or 3,000 j-ears, differed from that of the present. 

 Hence along many lines the study of the climatic variations of historic times is essential 

 as the foundation of future work. 



The present volume is an attempt to determine the sequence and character of such 

 variations on the basis of evidence in the drier portions of America from Guatemala on the 

 south to Idaho on the north. A large number of phenomena from the diverse fields of 

 geology, archeology, history, and botany seem to agree in indicating that during the past 

 3,000 years North America has been subject to pronounced climatic pulsations similar 

 to those which appear to have taken place in Asia and other parts of the Old World. In 

 the temperate portions of the Eastern Hemisphere the climate of the past appears on the 

 whole to have been distinctly moister than that of the present. The change from the 

 past to the present, however, does not seem to have been gradual and regular, but pulsatory 

 or cychc, so that certain periods have been exceptionally dry, while others have been wet. 

 In America the same appears to be true. 



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