376 BOTANICAL GAZETTE [MAY 
angle of slope varies from twenty to thirty degrees. When once 
a sweep is formed the tendency to self-perpetuation becomes 
greater and greater, since the wind becomes more and more con- 
centrated as the sweep grows deeper. The destruction of the 
forest vegetation is very soon accomplished at such a place. 
The desiccating influence of the wind becomes increased and 
Fic. 25.—Juniper heath on North Manitou island. Young heath patches = = 
left background. F ully developed heath in the foreground. Advancing coniferous 
forest at the right background. 
makes it difficult even for the xerophytic conifers to survive > 
no place is the destructive action of the sand-blast seen a z 
as in these rejuvenated sweeps. The branches and even . 
trunks of the trees have the softer parts carved away, — _ 
More resistant portions stand out in conspicuous renee ven 
leaves, especially of deciduous trees, are torn or withered oF © 
altogether destroyed. 
These destructive agencies are aided by another pao 
altogether irresistible when the sweeps grow deeper; the for 
hat is 
ce of 
