pe BOTANICAL GAZETTE [SEPTEMBER 
in the Crystal Lake Bar region (10) does not appear in this region, 
probably because the corresponding habitat is not present. This 
association contained Thuja occidentalis, Abies balsamea, Tilia 
americana, and Osirya virginiana, mixed with the trees of the climax 
deciduous forest on the crest of the bluffs facing Lake Michigan. It 
was apparently due to the exposure to sun and lake winds, combined 
with low soil moisture content. The only place where it might 
have been expected was on the high bank at the eastern edge of the 
o.—Morainic remnant on shore of Lake Michigan; laridward slope equally 
ed ‘wid hemlocks growing in valley. 
sand ridge region, and there the only conifers seem to have been 
Pinus Strobus and P. resinosa. 
The preceding discussion indicates that the migration of the 
beech-maple-hemlock forest into the sand ridge formation has been 
of two sorts, one a general advance along all lines of contact 
between the two formations, the other in a long slender belt on 
the morainic ridge and its remains, to the shore of Lake Michigan 
(fig. 10). Unpublished investigations by the writer indicate that for 
the establishment of Thuja occidentalis and Abies balsamea on sandy 
soil it is necessary that there should be present in the sand enough 
moisture to carry the young tree root below the drought zone. This 
