1918] HUTCHINSON—FOREST TREES 477 
changes; with respect to the latter migration has lagged behind. 
Upon such a basis the “anomalous” separation of the deciduous 
hardwood forest and the coniferous forest is readily explained. 
The granitic rock of the Laurentian Plateau has weathered slowly, 
humus has accumulated slowly; in brief, the soil has developed 
slowly, hence the migration of the climax forest has been checked. 
This principle applies regionally as well as locally. In the region 
of glacial moraines the deep soil has made possible a rapid accumu- 
lation of humus, as well as a thorough intermingling of rock soil 
and humus. There has been a rapid development of the soil, 
consequently the Acer-Fagus forest has been permitted to invade 
such regions. The time factor as an expression of the rate of soil 
development has limited the rate of migration and hence forest 
distributfon also. 
The time factor in relation to soil development explains both the 
numerous northerly outliers of Acer associations and also the 
outliers of the Picea-Abies forest. The northward deviation of 
the limits of such species as Acer saccharum, Fagus americana, and 
Tsuga canadensis has been noted. It is significant that this 
deviation coincides with a great depression extending to the height 
of land in which highly developed soil deposits are present. The 
deciduous hardwoods occur as outliers and are always found on 
the better soils. Although the writer has not been able to study 
the Saugenay basin personally, it may be ventured that the north- 
ward migration of Acer at this point is also to be explained on the 
basis of soil development. Bray (5) has found difficulty in explain- 
ing the occurrence of such trees as Abies and Picea in the swamps 
of New York. The soil in such localities is in a primitive stage of 
development. Although much humus has accumulated, there has 
been little or no intermingling of rock soil and humus, and the 
degree of aeration is low. The soil has been protected from the 
action of atmospheric agencies and running water, hence its 
undeveloped condition. The result is the same as when the 
Slowly weathering Laurentian rock resists the agencies which 
promote soil formation. The fact that the same forest species are 
present in both places emphasizes the potency of the rate of soil : 
development as a factor in the determination of tree distribution. 
