662 Plain, Prairie, and Forest. [ November, 
sota, admitting that the soil is coarser and more gravelly than it 
is on the adjacent prairies, although he sees no connection be- 
tween this peculiar character of the soil and the exceptional ex- 
istence of an extensive forest upon it, while at the same time rec- 
ognizing the dilemma in which he is placed by his adoption of 
the prairie-fire theory. The writer has often noticed, during his 
explorations just on the western edge of the Lead Region, that 
the vicinity of old, abandoned shafts was becoming overgrown 
with trees, the fact being that in the sinking coarser materials 
underlying the prairie soil had been reached and thrown out in 
abundance on the surface, and that it was on this gravelly detri- 
tus that the trees were growing, the adjacent, undisturbed prairie 
remaining in its natural, grassed condition. 
The distribution of the timbered and prairie tracts in Wiscon- 
sin, as already suggested, illustrates beautifully the dependence 
of the forest growth on geological conditions rather than on those 
having to do with climate. In the northern part of the State, 
as we see indicated on Professor Brewer's map, is a region of 
dense forest, although, as the table of rain-fall statistics given 
on a preceding page ! shows, this is not a region of large precipita- 
tion. It is, however, heavily covered with coarse detrital mate- 
rials, plentifully distributed from the “ head-quarters of the drift,” 
on Lake Superior. The rocks underlying the drift deposits are 
crystalline, belonging to the Azoic series, and the surface is rough 
and broken, being intersected with low ridges, and knobs of 
granite and trap. South of this is a large area occupying the 
central portion of the State and extending down as far as the 
Wisconsin River, almost exclusively occupied by a very pure 
silicious sandstone, which is wrapped about the Azoic region, eX 
tending in a northeasterly direction to the Menomonee River, and 
northwest to the Falls of the St. Croix. This great sandstone 
covered area is the pine district of the State, while south of the 
Wisconsin is the region of oak openings and prairies. And 
when we reach these treeless tracts, the range and extent of 
which have already been indicated, we find that we have got en- 
tirely beyond the drift-covered area, and that we are upon a 501 
made up of the insoluble residuum left from the disintegration “ 
several hundreds of feet in thickness of limestone and dolomites, 
which have been dissolved out and carried away by the rail» 
there being abundant evidence that this region has never bae 
covered by water since it was first raised above the Silurian 
ł See NATURALIST, X. 586. 
