406 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1»12. 



Prof. Douglass there shows that m dry regions the thickness of the 

 rings of wood formed each year varies so closely in harmony with 

 climatic conditions that it may be used as a measure of the climate of 

 the past. Taking about 20 sections of trees from two to five hundred 

 years of age, he made micrometer readings of the thickness of each 

 individual ring, and thus obtained the average growth for each year. 

 As a result he finds that the curves of growth and of rainfall for the 

 40 years or so since records are available present a marked agreement. 

 Manifestly, then, the curves of growth may be used as a means of 

 determinmg the ramfall hi periods long antecedent to any human 

 records. Douglass has so used them, and his analysis indicates 

 cycles of precipitation with a periodicity of 11, 21.2, and 32.8 years, 

 a result which agrees with the cycles indicated by records of rainfall. 

 He did not attempt to investigate larger cycles, nor to determine 

 the relation of the climate of periods separated by hundreds of years. 

 Obviously this is possible, and it needs only an expansion of Douglass's 

 method and a study of the sources of possible error to enable us to 

 come to definite conclusions as to the nature of the climate at any 

 time back to the youth of the oldest available trees. 



During the year 1911 I began to make use of the method of Prof. 

 Douglass and obtained some most mteresting results. In the first 

 place. Prof. H. S. Graves, Forester of the United States Forest 

 Service, most courteously placed at my disposal several thousand 

 "stem analyses," or measurements of the rmgs of trees by the members 

 of his bureau in various parts of the United States. In the second 

 place, I went to California on behalf of the Carnegie Institution, and 

 with several assistants made similar analyses of 451 of the *'Big 

 Trees " or Sequoia wasMngtoniana of the Sierra Nevadas. 



Let us first ascertain what may be learned from a study of the 

 trees of New Mexico which grow in the mountains close to the ruins 

 which we have hitherto been considering. The courtesies of the 

 Forest Service have included not only the placing of a large number 

 of stump analyses in my hands by the chief of the bureau, but also 

 some most helpful compilations by other members of the service. I 

 am especially indebted to Mr. A. B. Recknagel, chief of silviculture 

 at Albuquerque, N. Mex., who has gathered a most valuable series 

 of statistics for the yellow pines of that State, and has most gener- 

 ously allowed me to use them. His data are based on 645 trees from 

 four of the United States forest reserves, scattered in such a way 

 that they have approximately the same distribution as the ruins 

 with which we have been dealing. In general, however, they lie at 

 greater altitudes than the ruins, in the portion of the yeUow-pine 

 area where the trees grow best and where they are neither at the 

 lower limit so as to be especially hable to injury by drought nor at 

 the upper hmit so as to be especially liable to injury by excessively 



