RELATIONSHIPS OF METASPERMAE. 607 
variety of conditions. It becomes, therefore, both a condition 
and, to some extent, a cause of the diversity of plant-popula- 
tion. From the bare gneissic rocks of the Granite Falls 
district one may find in the valley all sorts of variations to the 
peat-bog soil of the Mankato and Kasota districts. Heresands 
or gravels in various proportions, there clays of different 
compositions or mixtures of sands and clays give a great diver- 
sity to the soil-surface of the basin. As, through water agency, 
the soil becomes more and more analysed and segregated as 
one leaves the general prairie level in passing down some 
channel, so in the main gorge at different places where different 
kinds of selective action have progressed and where the cumu- 
lative selective action of tributary streams is felt, may be 
found the maximum of variety in passing from one area to 
another. Itis possibly due in part to this relative homogeneity 
of the prairie soils and relative heterogeneity of the bottom- 
land soils that the prairie itself supports relatively a 
more homogeneous plant-population than the bottom- 
lands. The grasses, composites, pulses, polygalas, phlox and 
sedges of the rolling prairie constitute after all a rather smal} 
percentage of the total population of the valley. To contribute 
to this result not only past glacial actions, but present, con- 
tinuous activities of rain, wind and temperature have played 
their part. Itis, however, not incorrect to attribute, in part, 
the difference between the upland and lowland characters, to 
the glacial invasion. 
The topography of the valley is evidently in great part due 
to the subsequent action of the various forces of nature upon 
the general mass of till which was deposited in a sheet averag- 
ing more than two hundred feet in thickness over the older 
surfaces of the basin. The original aspect of this drift-sheet 
was doubtless somewhat undulating and under the weathering 
and erosion of the last 7,800 years it has come to present its 
modern aspect. The hills have become rounded, the streams 
have cut their gorges and deposited their silt in the form of 
alluvium, the lakes, formed by the disturbance of the old 
drainage, have sometimes persisted, with, however, reductions 
of original size in varying degree, and have sometimes disap- 
peared through the cutting and draining action of their ontlets 
or through the silting up of their inlets. Thus many level 
meadows have been formed and the production of such 
meadows from older lakes may be seen going on to-day. In 
this way, habitats are provided for aquatic plant-immigrants, 
