Postelstia 41 
mation according to its composition and after 
the manner of its subdivisions we have first to 
deal with 
a Forest of low altitude front- 
ing the sea (sea level to room) 
The width of this zone varies with the degree 
omcope. In those places where the slope is 
abrupt, conditions that characterize the higher 
levels are sooner met with than where the rise is 
eradual.. Elevation is apparently of more im- 
portance than distance from the sea. In shelter- 
ed places like within San Juan Harbor where 
wave action is not greatlv felt, the forest ap- 
proaches much nearer to the high tide mark 
than along the exposed shores. For this reason 
a great deal of the: vegetation characteristic of 
the rocky beaches is eliminated and one sees fre- 
quently the branches of large trees reaching out 
over the water. No one particular species of 
tree seems to form the forest vanguard. Picea 
stichensis, Thuja plicata and Tsuga heterophylla 
occur with about equal frequency, although in 
some places the hemlock and cedar yield to the 
spruce. When Alnus oregana occurs it is found 
