1908] HARVEY—PRAIRIE-GRASS FORMATION 87 
of physiographic development (BEssry ’99). And again, as the 
prairie existed as a climatic formation long before these fires, either 
of Indian or Caucasian origin, swept the prairies, it would appear that 
this fire factor has been overestimated by many and is in no sense to 
be regarded as the fundamental factor. That there is, nevertheless, 
an extremely slow advance of the forest usually through the medium 
of its forerunner, the shrub association, whose pioneer Symphoricar pus 
occidentalis is followed by Rhus glabra, is very evident. 
While the above factors are seemingly adequate to account for 
tree absence upon the western upland prairie, yet in light of the 
prairie as a natural climatic formation it would seem more proper to 
make the problem one accounting for the occasional presence rather 
than the general absence of trees. A future paper is planned to 
discuss the factors controlling this encroachment. 
The period of growth resumption 
Not until about the first week in March are climatological condi- 
tions at all favorable to an awakening of vegetation, and then only 
on infrequent days; but the month as a whole is marked by the open- 
=“s of flower and leaf buds of trees and the beginning of the germina- - 
ion of prevernal annuals and the formation of the basal rosettes of the 
Perennials, though some tide over from the previous season. The 
prevernal bloomers naturally make most rapid progress, aided by 
their geophytic habit. Not infrequently the temperature falls below 
freezing and killing frosts ordinarily result. Light snow storms, 
Which rarely occur, may temporarily retard growth. The conditions 
ome progressively more favorable and pass insensibly into the 
Period of the prevernal flowers. 
The usual snows and rains of the early part of the month assure 
abundant moisture. The chresard, which is about 18 to 20 per cent. 
m the early days of the month, decreases to about 14 per cent. at the 
middle, and to about 12 per cent, toward the last of the month, thus 
Siving an average chresard of some 15 per cent. The different 
©xposures of the prairie hills, knolls, and ravines progressively recover 
m the effects of winter in the following order: southeast to south- 
West, southwest to northwest, northeast to southeast, and northwest 
to northeast, The frost clings to the northern exposure in ravines, 
