INTRODUCTION. 6 



ancient human occupation. In this part of the work we shall examine a large number of 

 ruins scattered from the shore of the Gulf of California to the northern hmits of New 

 Mexico, 500 miles away. Except among a few archeologists, the number of ruins is rarely 

 appreciated. The ruins not only indicate a considerable degree of culture, but they show 

 distinctly that different races occupied the same sites in succession. The successive occu- 

 pations were separated by periods of abandonment, due possibly to chmatic causes, or 

 perhaps to something quite different, but at least well worthy of study. In considering 

 the ancient ruins and their prehistoric inhabitants it is essential to keep fairly in mind two 

 opposing theories. The first, which is usually accepted, holds that the large number of 

 ruins does not indicate a correspondingly large population, and that the physical conditions 

 of the country in the past, just as at present, forbade any great number of inhabitants. 

 The other, which is accepted by only a few scholars, holds that the ruins were occupied by 

 a relatively dense population which persisted for a long time. This could have been 

 possible only on the assumption of greater rainfall than now, and therefore those who 

 hold this view believe in changes of climate. 



The hues of reasoning followed thus far are similar to those which I have employed in 

 respect to various countries of Asia, and which were first set forth in a volume entitled 

 "Exploration in Turkestan," published by the Carnegie Institution of Washington in 1905, 

 and have been amphfied and revised in "The Pulse of Asia" and "Palestine and its Trans- 

 formation," published in 1907 and 1911, respectively. The conclusions derived from these 

 Unes of reasoning are open to the criticism that a preconceived theory may have led to 

 the interpretation of phenomena according to that theory. Hence, before the conclusions 

 here indicated deserve final acceptance, it is necessary to compare them with the results 

 obtained by the observations of other unprejudiced workers, or with the independent 

 results of some new method in which the personal equation plays no part. Fortunately 

 the work of Professor A. E. Douglass, of the University of Arizona, suggests a method by 

 which old trees may be used as a mathematical measuring-rod in order to determine 

 exactly what chmatic events have occurred during the last 2,000 or 3,000 years. Accord- 

 ingly, considerable space will be devoted to setting forth the results of the measurement 

 of the rate of growth of nearly 500 Sequoia trees among the Sierra Mountains in CaUfornia. 

 A large number of other measurements of trees by the United States Forest Service have 

 been kindlj' put at the disposal of the Carnegie Institution of Washington by the Forester, 

 Mr. Henry S. Graves, and a discussion of them is included in this volume. By purely 

 mathematical methods, unaffected by any personal bias, it has been possible to obtain 

 curves indicating the climatic pulsations of the last 3,000 years. A comparison of these 

 curves with the results obtained from other lines of evidence, both in America and Asia, 

 shows that in spite of certain disagreements the general chmatic history of both continents 

 appears to have been characterized by similar pulsations having a periodicity of hundreds 

 or thousands of years. 



If the conclusions outhned above are accepted, they may perhaps furnish a key to the 

 pre-Columbian chronology of America. Apparently the Southwest has been first relatively 

 inhabitable and then relatively uninhabitable during periods lasting hundreds of years. 

 The dates of these periods are ascertainable from ancient trees. Each propitious period 

 has probably been a time of expanding culture and comparatively dense population, while 

 the unpropitious periods have been times of invasion, disaster, and depopulation. Archeo- 

 logical study has begun to differentiate periods of this sort, but has been unable to date or 

 correlate them. If the evidence of climate be considered together with that of archeology, 

 we may perhaps at length be able to overcome the absence of written documents so far as 

 to construct a fairly intelligible record of the history of our ancient predecessors. 



The broader the area under observation, the greater is the probabihty of accurate 

 results. Hence, after two field seasons in Arizona, New Mexico, and Sonora, it seemed wise 



