1918] HUTCHINSON—FOREST TREES 473 
humus content of the soil proper is low. In the lowlands a similar 
condition maintains. This is especially applicable to the peat bog, 
where humus is most abundant. There has been no movement, 
however, of the particles of the contiguous strata of the rock soil 
and the overlying humus; they are distinct, hence the otherwise 
valuable humus is practically useless in so far as the improvement of 
soil properties is concerned. On the ridges which were exposed 
first by the subsidence of glacial ice and water there is much deeper 
soil, and the humus: accumulated from antecedent vegetation has 
become intimately associated with the rock soil by weathering. 
The soil proper has a high humus content and is able to support 
such trees as Acer and Fagus. It will be remembered that the - 
temperature, precipitation, humidity, and original soil composi- 
tion may be regarded as constant; the varying factors are those 
associated with the accumulation of humus. The humus content 
of the soil is at least a local factor in the determination of tree 
distribution. : 
It has been maintained that differences in the composition of 
soil have only a local effect. It does not seem clear why a factor 
which is potent locally should not be potent throughout greater 
areas: The gradients of soil changes are usually greater when 
limited areas are considered; hence also those of the associated 
floral changes. The Laurentian Plateau is a great area dominated 
by the coniferous forest, while the contiguous region of glacial 
drift is dominated by the deciduous hardwood forest. The marked 
differences in forest species prevailing in the regions north and south 
of the Kawartha Lakes, respectively, is strikingly in accord with 
soil differences. Moreover, the line separating the dominantly 
coniferous region from the dominantly deciduous hardwood region 
does not follow any isotherm or the boundaries of any precipitation 
area, but rather the outlines of the Laurentian Plateau, roughly 
from the southeast part of Georgian Bay to Lake Simcoe along 
the Kawartha Lakes, southeastward to the Thousand Islands, 
northward to Ottawa, and again eastward along the northern limits 
of the Ottawa and St. Lawrence valleys. In the coniferous region 
there are oases of deciduous hardwoods of considerable area, such 
as that at Renfrew, or of limited area, such as the ridges already 
mentioned; in fact, wherever the soil is similar to that found in the 
