50 IOWA ACADEMY OF SCIENCES. 



accounted for in the manner suggested by a succeeding portion 

 of this paper, but nevertheless it is not wholly wanting in 

 value as testimony in support of the fire theory. 



2. Excess of Moisture. — That excess of moisture is destructive 

 to trees has long ago been established, and Lesquereaux, 

 White and others, contended that such excess was primarily 

 responsible for our prairies.* 



They argued in substance, that the regions now occupied by 

 prairies were formerly, after the recession of the glaciers, 

 large lakes which gradually became swamps, and then dried, 

 forming prairie which remained comparatively damp, the soil 

 becoming ' * sour, " because of poor drainage. Trees do not 

 prosper in such soils, and this theory may be of value in 

 explaining the absence of trees from portions of the drift plains 

 of north-central Iowa, and from local low tracts, but it is not 

 tenable for the loess hills of western Iowa, nor indeed for the 

 rougher treeless parts of the drift area, such as those in west- 

 ern Lyon county. Excessively wet seasons may also be con- 

 sidered among the conditions unfavorable to the extension of 

 forest areas, while in the same connection the effect of such 

 seasons upon the fungus and insect enemies of trees, should 

 receive consideration, 



3. Insufficient Moisture. — That the amount of rainfall in Iowa 

 diminishes as we go northwestward is a well-known fact. That 

 the amount of forest varies in somewhat the same manner has 

 also been pointed out, f and may be readily observed by refer- 

 ence to the appended map. The diminution in rainfall nat- 

 urally produces conditions unfavorable to the growth of trees, 

 and this variation in amount may account for some of the dif- 

 ferences between the forest conditions of the northwestern and 

 other portions of the state. It does not, however, account for 

 the differences which we may observe in either of these sec- 

 tions. It does not explain why we have prairie tracts in the 

 eastern part of the state, and groves in the western part, though 

 the fact that the northern and eastern slopes in all parts of the 

 state are more likely to produce groves, because they are more 



*Oaleb Atwater, In Am. Jour. Scl. and Arts, 1st series, Vol. I, p. 120, 1818; A. Bourne, In 

 same, Vol. II, p. 86, 1820; W. W. McGulre, in same, Vol. XXVI, pp. 93-8; reprint In same 

 Vol. XXXIII, p. 1839; Henry Engleman, In Am. Jour. Scl. and Arts, 3d series. Vol 

 XXXVI, p. 384, 1863; Alex. Wlnchell, In same. Vol. XXXVIII, p. 332, 1864; Leo Lesquer 

 eaux. In same. Vol. XXXIX, p. 317, and Vol. XL, p. 23, 1865; Jas. D. Dana, In same, Vol 

 XL, pp. 293 et seq., 1865; O. A. White, In Am. Nat., Vol. II, pp. 143-155, 1868; J. D. Whit 

 ney, in Am. Nat., Vol X, pp. 656 etseq., 1876. 



tFirst Annual Rep. of Iowa Weather Station, p. 50. 



