52 IOWA ACADEMY OP SCIENCES. 



5. Geological formations and soils. — To the superficial geolog- 

 ical formations and soils have been attributed various 

 influences upon the development of forests and prairies.* 



Some of the earlier observers reported that the loess was 

 unfit for the growth of trees, but McGee showed, and now 

 everyone recognizes, that in northeastern Iowa it is the tree- 

 producing formation, while the drift is almost treeless. But the 

 loess of the western part of the state is largely treeless, while 

 groves are found on the drift in Worth, Winnebago, Dickinson, 

 and other counties of the state. In the latter, however, there 

 is always a thin veneer of fine loess-like soil, f 



Moreover, alluvium everywhere may or may not produce 

 native trees. It is, therefore, evident that while the fineness 

 and quality of the soil no doubt influence the growth of trees, 

 the particular geological formation or soil does not uniformly 

 determine such growth. It may, however, produce some effect 

 by developing upon different soils unequal quantities of fuel 

 for destructive fires. X 



The foregoing are the most common causes and conditions 

 which have been cited in explanation of the prairie phe- 

 nomena. There remains one more which has thus far received 

 rather scant notice, § but which deserves a high rank, namely: 

 wind. The effect of wind upon trees may be twofold — mechan- 

 ical and physiological. The first is produced by breaking 

 branches or even trunks of trees; by stripping or injuring foli- 

 ao-e, by driving sand and dust against the more delicate tissues, 

 into the stomata, etc., and by spreading fires. It was evidently 

 this that Whitney had in mind when he wrote :11 "If the force 

 of the wind were essentially inimical to the growth of trees, 

 we should find them thriving, if anywhere, in the sheltered 

 nooks, and to the leeward of the northwesters, that being the 



*See: J. D. Whitney in Hall's Geology of Iowa, Vol. I, pt. I, p. 24, 1858; In A.m. Nat., 

 Vol X pp 577-656, 1876; W. J. McGee In Proc. Am. A. Ad. Sci , Series I, Vol. XXVII, p. 

 198, 1878; in Pop, Sci. Mo., Vol. XLIX, p. 115, 1883; Thos. J. Howell in Pop. Sci. Mo., Vol. 

 XXIII pp 531-3 1883; W. J. McQee in Eleventh Annual Rep. U. S. Geol. Sur., pp. 298-8, 

 1891;L.'h. PammelinlowaGeol. Sur., Vol. V, p.233, 1895; Thos. H. Macbride in Proc. 



Iowa Acad. Sci., Vol. Ill, p. 96, 1896. „ , rxr 



tSee author's discussion of surface deposits, in Proc. Iowa Acad. Sci., Vol. IV, pp. 



69 et. seq., 1897. „ , , 



tSee: R. W. Wells in Am. Jour. Sci. and Arts, Series I, Vol. I, p. 333, 1818; and for 

 more complete discussion, Thos. H. Macbride in Iowa Geol. Sur., Vol. IX, pp. 148-9, 1898. 



§ See Dr. Rush Nutt In Am.' Jour. Sci. and Arts, series I, Vol. XXIII. pp. 40-45, 1833; 

 paper read by Prof. H. H. McAfee, before the Am. For. Ass'n at Philadelphia, Sept., 

 1876; Rep. of U. S. Dep't Agrl. for 1889, p. 276; Rep. of Chief of Dlv. of For. for 1891. p. 207; 

 Bull. Div. of For., 1893, p. 119. 



II Am. Nat., Vol. X, p. 583. 1876. 



