November 19, 1909] 



SCIENCE 



723 



such are usually marked with great definite- 

 ness and bear no relation to aspect. Moreover, 

 these sharp boundary lines coincide through- 

 out with the more plainly marked surface con- 

 tacts and boundaries of such geologic forma- 

 tions. Cases occur in which, if two quadrats 

 of 1,000 square feet each were laid off, each 

 on a separate formation,'" but adjoining one 

 another on one side and on the contact line, 

 not a single woody species would be common 

 to both quadrats, although several such might 

 be found on either.""" The first class of areas 

 is composed of limestone, the second and 

 smallest in total extent of basalt, and the third 

 and by far the largest, of older igneous and 

 metamorphic rocks, predominant among which 

 are andesite, rhyolite, granite and quartzite." 



The second division is practically confined 

 to certain volcanic outbursts in the eastern 

 and southern parts of the range, but certain 

 spots also occur on its west side. The com- 

 paratively recent origin of these is shown by 

 the fact that on the rim of a former crater 

 were found volcanic bombs in a state of ex- 

 cellent preservation and fragments of lava 

 that still bore plainly the marks of former 

 plasticity. Grasses Eind herbs cover these hills, 

 but they are characterized by the almost com- 

 plete absence of tree and shrub growth. The 

 adjacent hills of andesite and rhyolite bear 

 with the same aspect and general altitude of 

 5,000 feet the usual evergreen oaks and juni- 

 pers. Whether comparative age of the forma- 

 tion is a factor or not, the cause plainly lies 

 in the substratum. 



The first division is distinguished from the 

 great composite third mainly by species of 



" The term " formation " is here used only with 

 reference to rock and soil. 



" In Europe, these definite floral boundaries 

 have long been known to occur, and their imme- 

 diate cause has been recognized. See Warming, 

 " Pflanzengeog.," sec. ed., p. 78. 



" MacDougal, Plant World, XI., p. 270, 1908. 



" Not included in above divisions, were also 

 found smaller bodies of volcanic tuff, and still 

 more infrequently, sandstone and shale. For the 

 age of some Chiricahua formations see E. T. 

 Dumble, " Notes on Geology of S.E. Arizona," 

 Trans. Am. Inst. Min. Eng., Feb., 1901. 



smaller stature and leaf surface, i. e., more 

 distinctly zerophytic chaparral character. The 

 following collection of woody species, growing 

 on a steep, westerly, limestone slope near 

 Hands' Cabin at approximately 7,000 feet, 

 may serve as a type for calcareous societies 

 of similar situation. The species are ranked 

 in order of abundance: 1. Ceanothus greggii 

 A. Gray. 2. Gercocarpus hreviflorus A. Gray. 

 3. Ehus virens Lindh. 4. Garrya wrightii 

 Torr. 5. Pinus cenibroides Zucc. 6. Juni- 

 perus pachyphlcea Torr. Below is a society of 

 trees and shrubs typical of a habitat similar 

 to that above in all chief particulars except 

 rock and resultant topography and soil, which 

 is andesitic : Quercus arizonica Sarg., Q. hy- 

 poleuca Engelm., Q. reticulata H. B. K., Pinus 

 chihuahuana Engelm., P. mayriana Sudw., 

 Ceanothus fendleri A. Gray, Gymnosperma 

 corymhosa DC, Juniperus pachyphlma Torr. 



A number of fern species occur upon the 

 limestone, very few upon the andesite, but the 

 latter often supports a better grass cover. 

 Dasylirion Wheeleri Wats., though present 

 elsewhere, is highly characteristic of limestone, 

 Nolina erumpens Wats, and Yucca macro- 

 carpa (Torr.) Coville are seen at their best 

 on non-calcareous soil. The seven species of 

 evergreen oak are almost absolutely absent 

 from pure limestone soil. The only oak 

 (Quercus pungens Liebm.) on limestone is not 

 evergreen, and this is never found on other 

 rocks of the region covered. 



In a notable recent paper," Fernald brings 

 out similar great contrasts between the alpine 

 floras of northeastern America, and their 

 direct relation to the rock and soil on which 

 they grow. As is abundantly the case in the 

 Chiricahuas, he finds plants that are very 

 definitely limited to certain residual soils on 

 slopes and table-lands, freely commingling on 

 the mixed elements of transported soils at the 

 foot of slopes and along the watercourses. 



The ultimate causes of these distributional 

 phenomena, apart from those controlled by 

 altitude and aspect, thus plainly lie in the 

 substratum. How much is due to historic 

 determinants, and how much to physicochem- 



" Cont. Gray Herb., Harv. Univ., X. S., XXXV., 

 1907. 



