1905] TRANSEAUBOGS OF THE HURON RIVER VALLEY 419 



than the forests of mineral soils. These phenomena have not been 

 observed in any of the bogs in this vicinity. 



The same statement holds for the presence of loose floating bogs 

 which are driven about on lakes by winds (35). 



2. Temperature. In its temperature relations both the topography 

 and the character of the substratum combine to influence the bog 

 habitat. It has long been noted by agricultural writers that reclaimed 

 bog areas are particularly subject to late frosts in the spring. One 

 of the causes of this peculiarity lies in the fact that on clear and quiet 

 nights the cooled air overlying elevations drains into the depressions 

 (n). Some recent observations made by SEELEY (45) near Chicago 

 show how effective such atmospheric drainage may be even in dis- 

 tricts whose range of elevations amounts to but 15 feet (4.5 m ). He 

 found that the hilltop averaged, on the night of the observations, 

 2.5 F. (1.4 C.) higher than that of the depression while a ther- 

 mometer placed 30 feet (9) above the hilltop averaged 8.8 F. 

 (5 C.) above that of the " swale." On comparing the temperatures 

 of atmospherically undrained and drained depressions with that of 

 the hilltop, he found that the hilltop temperature was 36 . 3 F. when 

 that of the drained depression was 36 F. and that of undrained 

 31.8 F. Here is a particular instance in which frost occurred in 

 the undrained depression, but not in the other situations. On quiet 

 nights low grounds in general are subject to lower temperatures 

 than the adjoining highlands, and it is probable that these effects 

 are more pronounced in the case of undrained depressions. 



A second factor in the production of low temperatures in bogs 

 is found in the nature of the substratum. In the spring the ice which 

 has formed beneath the cassandra and tamarack areas melts with 

 extreme slowness, when once the surface of the soil has been reached. 

 This is explained by the low conductivity of the loose, partially 

 decayed, vegetable covering, and by the shading of the plants above. 

 For example, at First Sister Lake, in 1904, the ice had disappeared 

 from the water surface on April 10. On April 17, with an air tem- 

 perature of 10 C., the temperature of the substratum in the bog 

 sedge zone averaged 10 C., in the Cassandra zone 6 C., in the 

 tamarack zone 3 C., and the area of willows and sedges 8 C. Ice 

 was found at several points among the tamaracks, an inch below the 



