210 NATUEAL HISTOEY BULLETIN 



low winter rainfall. Several authors recognized drouth as an 

 accessory factor, chiefly in connection with prairie fires. A few 

 writers ventured to deny the importance of this factor, or to 

 give it a secondary place. Among these are Whitney (1876, 

 1882), Tarr (1896) and the writer (1900). 



Kainfall is naturally an important factor in determining the 

 amount of moisture available for the use of plants, and it also 

 influences the rate of evaporation, but it does not explain the 

 characteristic intermingling of prairie and forest in the Missis- 

 sippi valley, nor the ocurrence of prairie and forest on opposite 

 sides of a sharp ridge, nor the presence of small prairie tracts 

 (barrens or oak-openings) in the heart of wooded areas, for in 

 all these cases the amount of rain falling upon contiguous prairie 

 and forest is the same, and the very unequal amount of available 

 moisture upon the two kinds of surfaces cannot possibly there- 

 fore be determined by the amount of precipitation. 



Moreover the annual amount of rainfall within the state of 

 Iowa, more than seven-eighths of the surface of which was prairie, 

 is quite sufficient to sustain a forest growth. This is amply 

 demonstrated by the remarkable record of meteorological obser- 

 vations made at Logan in the relatively drier western part of 

 the state by Mr. Glenn H. Stern and his grandparents, which 

 extends over a period of 43 years, from 1867 to 1909 inclusive, 

 A summary of this record^*' is here presented, the rainfall being 

 given in inches for each year. 



Annual precipitation of moisture, at Logan, Iowa 



from 1866 to 1909. 



1867 — 27.81 1878 — 46.31 1889 — 29.87 1900 — 31.39 



1868 — 29.85 1879 — 33.10 1890 — 34.95 1901 — 30.56 



1869 — 44.95 1880 — 27.30 1891 — 35.39 1902 — 40.74 



1870 — 25.30 1881 — 56.60 1892 — 35.25 1903 — 30.25 



1871 — 28.95 1882 — 37.30 1893 — 22.40 1904 — 24.14 



1872 — 32.10 1883 — 39.90 1894 — 16.63 1905 — 30.35 



1873 — 43.20 1884 — 36.60 1895 — 26.12 1906 — 38.05 



1874 — 28.40 1885 — 40.20 1896 — 43.82 1907 — 22.73 



1875 — 42.00 1886 — 23.10 1897 — 26.00 1908 — 28.12 



1876 — 28.20 1887 — 23.60 1898 — 24.96 1909 — 43.39 



1877 — 45.10 1888 — 34.02 1899 — 31.95 



i6The record was published in detail by the writer in the report on Harri- 

 son and Monona counties, Iowa Geological Survey, vol. XX, 1910. 



