578 METASPERMAE OP THE MINNESOTA VALLEY. 



The climate is a characteristic inland North American one. 

 The rainfall has its source almost entirely in the Gulf of Mexico 

 vapors which extend up the Mississippi valley, precipitating 

 themselves in less and less amount as they pass from the 

 mouth of the Minnesota river to its headwaters. A summer 

 maximum of precipitation is conspicuous. The temperature 

 is, of course, less than that of points on the same x^arallels in 

 the old world, such as Marseilles, Florence or Odessa. It pre- 

 sents conspicuous summer maxima and winter minima. — 40° 

 Fahr. is reached occasionally during the months of January or 

 February, and — 30° Fahr. frequently. In the summer. 95° Fahr. 

 is reached occasionally and 90^ Fahr. during almost every 

 summer. The variations are more excessive in the western 

 and prairie regions than in the eastern and forest regions, 

 where the range of maxima and minima is least. The shelter- 

 ing effects of the forest, the lower altitude, and possibly the 

 smaller width of the valley, by interfering with cold or hot 

 winds, have much to do with this difference. The snow-fall is 

 greatest in the northwestern portion. The first frosts usually 

 occur near the middle of September, and the ice breaks up in 

 the streams late in March or early in April. 



Phaenological observations. No reliable phaenological ob- 

 servations have been made in the valley of the Minnesota. In 

 general, as elsewhere in the northern hemisphere, the plants 

 of the northern range are first to flower, and those of most 

 southern range last. The time of flowering of any species is a 

 useful indication of its history, and observations along this line 

 are much needed. The first shrub to put forth leaves is usually 

 Ai'temisia clracunculokles , the first trees to flower, the various 

 species of Salix. In the autumn a characteristic composite- 

 flowering is seen in the golden- rods, asters and sunflowers of 

 both prairie and forest region. The mid- summer season finds 

 the prairies gay with the purple of Laciniaria and Astragalus or 

 Spiesia. The oaks are usually among the last trees to drop 

 their leaves, and the cottonwoods among the first. Among 

 herbaceous plants chlorophyll persists the longest in the 

 grasses. 



Geological history of the Minnesota valley. The Minne- 

 sota river of today occupies an ancient gorge which was evi- 

 dently formed previous to the Cretaceous period, for Creta- 

 ceous clays are found in eroded hollows of the Shakopee lime- 

 stone in Le Sueur, Nicollet and Blue Earth counties and Cre- 

 taceous sandstones, clays and shales in the Cottonwood and 



