NO, I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 275 



tioned by rainfall, undergoes large and frequent fluctuations from the 

 wilting coefficient to field capacity at the extreme lower forest border. 

 Toward the forest interior these fluctuations subside until, finally, soil 

 moisture remains sufficiently constant to produce uniform sequences 

 and to tend toward unity of the annual increment. 



Although the present work has dealt specifically with tree growth 

 of the extreme lower forest border on the High Plains of Texas, sec- 

 tions were also obtained from trees growing under plentiful moisture 

 and short seasons (pi. 3, fig. 2) and from trees growing under very 

 low moisture and long seasons (pi. 36, figs, i and 2). Thus the sec- 

 tions at hand illustrate a range of conditions from high elevations in 

 New Mexico to the desert (and irrigation) at Yuma, Ariz. 



Those workers (Douglass, 1931, pp. 306-307; 1936, pp. 11-14; 

 1937, pp. 14-19; Antevs, 1948, pp. 168-169) who use growth layers 

 to date prehistoric events believe that cross-dating, which exists in its 

 most beautiful form in the lower forest-border trees of the Southwest, 

 proves the annual nature of the individual, sharply bordered growth 

 layer and also proves rain to be the controlling growth factor. Grow- 

 ing conditions may be such during a season that the following se- 

 quence occurs in some trees : formation of a sharp, complete, entire 

 growth layer; dormancy of cambium; reinitiation of cambial activ- 

 ity ; formation of a very thin, entire growth layer or a lens ; and ces- 

 sation of growth. In other trees the second diameter flush does not 

 occur. If the above sequence occurs, it seems reasonable that the thin, 

 entire growth layers or lenses would cross-date among those trees con- 

 taining them. They would be designated as "missing" from the re- 

 mainder of the trees. The fact that the thin, entire growth layers or 

 lenses match from one tree to another does not prove their annual 

 character. 



Uniform sequences of the forest interior do not lend themselves 

 qualitatively to visual cross-dating. Here, rainfall must be close to the 

 optimum amount insofar as visual differences in growth-layer thick- 

 nesses are concerned. Where rainfall drops below the optimum peri- 

 odically, variability of growth-layer sequences increases, and cross- 

 dating becomes possible. The farther and the oftener it drops below 

 the optimum, within limits, the more obvious and the more perfect 

 the cross-dating. But below the optimum is precisely where multiplic- 

 ity can, and does, occur. Excellent cross-dating and multiplicity, thus, 

 go hand-in-hand, at least in the lower forest border. 



The thought stemming from the evidence at hand may be put 

 another way. The more extreme the forest-border conditions are, 

 along with thin soil and steep, rocky slopes (Antevs, 1948, p. 168), 



