6 BOTANICAL GAZETTE [JANUARY 
mainland coast. These indicate periods when the water level was 
stationary for a considerable time. According to LANE (36) the 
present shore line is more strongly marked than any at higher 
levels. “Nor is it surprising,’ LANE remarks, “that the lake 
level should now be tolerably constant, for Lake Superior now 
drains over a rock: threshold.” In comparatively recent post- 
glacial time (since the formation of the very recent Nipissing beach) 
tilting occurred in the Lake Superior region, with uplift northward. 
This must have modified more or less the drainage conditions 
upon Isle Royale. It is important to bear in mind the self-evident 
fact that never since its first emergence from the waters of Lake 
Duluth has Isle Royale been connected with the mainland. 
For further geologic and physiographic details the reader is 
referred to LANE’s report (36), to which I am indebted for the 
material for this brief sketch; and also to ADAMs (4), who discusses 
the physiographic history of the island with considerable fulness. 
ApAms also gives much valuable data concerning the influence of 
the lake storms and surface currents upon the biota of the island. 
PHYSIOGRAPHIC AGENCIES NOW AT WORK 
The agencies that are now modifying the surface of the island, 
which are of course the same that have been active throughout 
its history, may be considered under two heads. 
Among the DESTRUCTIVE agencies, weathering is of the greatest 
importance in its influence upon vegetation. It is most evident 
upon the steep northwest slopes of the higher ridges. Here there 
are somewhat extensive talus piles lying at the bases of cliffs, or 
in some cases occupying the whole slope, the cliff having been 
buried by the accumulation of fragments. In many places the 
talus is fully clothed with climax forest, in others the fragments 
are bare or merely lichen covered. The results of weathering are 
evident also upon the bare rock shores, where scales and plates are 
seen to have been split from the rock surfaces through the agency 
of temperature changes. Very important, though effectually 
concealed, is the chemical action which is going on beneath the 
humus carpet that covers most of the island’s surface. Between 
the humus and the bed rock there is nearly everywhere a layer of 
