2 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID A]VIERICA. 



The facts set forth in this volume are the result of investigations carried on in coopera- 

 tion with the Department of Botanical Research of the Carnegie Institution of Washington, 

 at the invitation of the Director, Doctor D. T. MacDougal. The first field season, March, 

 April, and May, 1910, was devoted to the study of a relatively restricted area centering 

 at Tucson, the site of the Desert Botanical Laboratory in southern Arizona, and extending 

 southwestward for 150 miles to the shores of the Gulf of California in northwestern Mexico. 

 During the second season the months of March and April, 1911, were devoted to the 

 investigation of selected sites in various parts of New Mexico, while May and June were 

 occupied with measurements of the rate of growth of about 200 Sequoia trees in the central 

 parts of the Sierra Nevada Mountains in CaUfornia. The third season was divided into 

 two portions. In the first place, six weeks during March and April, 1912, were spent among 

 the lakes and ancient ruins of southern Mexico and Yucatan. In the second place, the 

 work upon the Sequoias in California was carried further, and about 250 more trees were 

 measured. Finally, in March and April, 1913, independently of the Carnegie Institution, 

 the author made a journey to Guatemala for the purpose of investigating the ruins of 

 that region and their relation to the physical surroundings and vegetation of the country. 



Throughout the work much attention was given to the influence of the present climatic 

 conditions upon physiography and upon the habits and distribution of plants and animals, 

 including man. No attempt will be made to deal with these subjects here, however, except 

 in so far as they bear upon changes of chmate. The purpose of this volume is primarily 

 to investigate the extent and nature of such changes and our attention will be devoted 

 almost exclusively to that subject, while the interesting problems of the relation of climate 

 to human character and history will be left for another volume. The first portion of our 

 investigations will be concerned entirely with New Mexico, Arizona, and the adjacent 

 parts of the Mexican state of Sonora. Inasmuch as an intelligent knowledge of present 

 climatic conditions is essential to a full understanding of the past, the present chmate of 

 those regions and its relation to the great climatic zones of the earth as a whole will form 

 our first subject of consideration. A minute knowledge of the land forms of the region 

 is not necessary for our present purpose, but a clear conception of the main types and 

 of their relation to climatic conditions is advisable. Accordingly a chapter will be devoted 

 to the general aspect of New Mexico, Ai-izona, and Sonora, and to the chief types of physio- 

 graphic forms commonly found there. To complete the picture a brief chapter on vegeta- 

 tion will be added, not from the point of view of the botanist but of the geographer. 



Having gained a general idea of the physical aspects of New Mexico and Arizona, we 

 shall be prepared to turn to some of the details which furnish e\adence as to past climatic 

 conditions. First we shall take up those hnes of inquiry which involve only physical 

 processes without respect to man. Among these a foremost place is occupied by the 

 alluvial terraces which are so widely distributed throughout all arid mountainous regions. 

 Their importance as possible indicators of chmatic changes is so great that I shall consider 

 the problem of their formation in detail. The strands of ancient lakes are often closely 

 associated with river terraces, and the conclusions to be drawn from them are similar. 

 Our region has so few lakes and these few have been so httle investigated that they are of 

 less importance than many other hnes of evidence. Nevertheless, the Otero soda lake 

 near Alamogordo in New Mexico and the group of small lakes near the City of Mexico are 

 of such interest as to warrant a somewhat full discussion. The desiccated bed of the Otero 

 Lake presents evidences of a changing climate not only in its old strands, but in the re- 

 markable series of gypsum dunes of various ages which surround it ; while in the Mexican 

 lakes natural causes appear to have induced variations in size even during the period since 

 the coming of the Spaniards. 



From the purely physiographic portion of our investigations in Arizona, New Mexico, 

 and old Mexico, we shall proceed to the main phase of the subject— the study of traces of 



