THE CORRECTION AND COMPARISON OF CURVES OF GROWTH. 137 



in both curves. In other words, when the rainfall of Idaho increases that of New Mexico 

 decreases, and the reverse, which is exactly the same thing that takes place in the growth 

 of the trees. The way in which the two sets of curves representing cUmate and growth agree 

 in presenting opposite phases in the two areas goes far toward proving that the method of 

 Professor Douglass can be relied upon. 



Before leaving this subject I would call attention to the fact that while the precipitation 

 of New Mexico is predominantly of the monsoon, summer, or continental tyije, and that 

 of Idaho is of the more northerly type, dependent on cyclonic storms and falling chiefly 

 in winter, both are distinctly of a mixed type. The opposition of the two curves harmon- 

 izes with the opposition of the summer and winter rains at Tucson as described in Chap- 

 ter I. In Chapter XVI, on the shifting of climatic zones, we come to the conclusion 

 that changes of climate are probably characterized by an alternate strengthening and 

 weakening of the earth's winds by reason of an increase or decrease in the intensity of baro- 

 metric pressure. Such a strengthening of the winds would at first thought seem to cause 

 an increase or decrease of both summer and winter rainfall at the same period. This, 

 however, does not seem to be the case in the minor variations now under consideration, 

 although it appears to be true when we come to the larger variations discussed in Chapter 

 XVI, where the shifting of clunatic zones is considered. Possibly this discrepancy is due 

 to the movement of Arctowski's "pleions" and "antipleions," or areas of excess or deficiency 

 of temperature, rainfall, and so forth. These, as will be shown more fully in Chapter XIX, 

 move back and forth in short periods. They are located chiefly within the Umits of the 

 United States and seem to be a purely continental phenomenon. Another possibility is 

 that Idaho lies so far north that the shifting of zones, wliich is discussed hereafter, brings 

 it into what is now the far northern region of hght rainfall at a time when New Mexico 

 comes into an area of heavy rainfall. As yet the matter is so little understood that any 

 satisfactory explanation is impossible. 



As a final method of testing the value of the curves discussed in this chapter, let us 

 see how the curve for New Mexico agrees with the conclusions which we have already 

 reached on the basis of the evidence of terraces, archeology, and history. In this con- 

 nection I would emphasize the fact that these conclusions were all reached before the 

 trees had been investigated. They have been set down in previous chapters exactly as 

 they were reached at a period of from 5 to 15 months before the tree measurements were 

 investigated. The importance of this lies in the fact that the agreement of the mathe- 

 matically derived tree curves with the conclusions derived from entirely different methods 

 furnishes strong confirmation of the accuracy of those methods as employed not only in 

 America but in Asia and southern Europe. 



From the ruins of Gran Quivira, it will be remembered, we concluded that at the time 

 of the Spanish occupation of New Mexico in the first half of the seventeenth century 

 chmatic conditions were distinctly more favorable than at present. About 1672 the ruins 

 of Gran Quivira seem to have been finally abandoned at the tune of the Pueblo rebelUon. 

 During the succeeding century conditions appear to have been somewhat better than 

 during the one that ended in 1900, as appears probable from the ruins of Buzani and one 

 or two other places not here mentioned. Let us compare the course of events thus indicated 

 with the final curve of growth of the yellow pines of New Mexico as derived from 272 trees 

 and as shown in the dotted line of figure 32. The high parts of the curve, according to 

 the conclusive investigations of Professor Douglass, point to moist conditions, and the 

 low to dry. If we interpret the curve in this way, it appears that perhaps one reason 

 why the Spaniards were able to establish themselves in New Mexico almost without a 

 blow was that from 1600 to 1650 the amount of rain and the general conditions of the 

 growth of vegetation were not only more favorable than now, but were becoming better 

 from year to year. Places like Gran Quivira were readily habitable and were growingly 



