660 Plain, Prairie, and Forest. [ November, 
northern drift phenomena have been displayed on a grand scale. 
Consequently, almost the whole of this area is covered with heavy 
deposits of coarse gravel and bowldery materials. ‘These deposits, 
if not at the surface, are near it, and the finer materials deposited 
on them, by alluvial and other agencies, generally form only a 
thin covering for the coarse deposits beneath. But as we go 
south and west from the region-indicated above, we find the un- 
derlying rock — the “ bed-rock,” as the Californian miners would 
call it — deeply covered with loose materials, it is true, but we 
observe also that these are quite different in character from what 
they are to the north and east. We come to a region where the 
drift agencies have been very limited in their action. The bulk of 
the superficial detritus has been formed from the decomposition of 
the underlying rock, and this detritus has been but little disturbed 
or moved from its original position. If erratic deposits exist, they 
are usually deeply covered with finer materials derived from close 
at hand. A great area exists in Wisconsin and Minnesota over 
which not a single drift pebble has ever been found, either at the 
surface or at any depth beneath it. The strata have become 
chemically disaggregated and dissolved by the percolation of the 
rain through them, the calcareous matter has been carried off in 
solution, and there is left behind as a residuum the insoluble 
matter which the rock originally contained, and which, consisting 
largely of silica and silicate of alumina, forms by its aggrega- 
tion a silicious and clayey deposit of almost impalpable fineness. 
It is this fine material which makes up the bulk of the prame 
soil; and, as the writer conceives, it is this fineness which is es- 
pecially inimical to the growth of trees. Exactly as we see the 
desiccated lakes in the midst of the forests gradually filling up 
with finely-comminuted materials and becoming covered with a 
growth of grasses or sedges, which is not afterwards encroached 
on by trees, no matter whether the ground becomes completely 
dry or whether it remains more or less swampy, so we have the 
prairies, which have certainly never at any time been overspread 
with forests, and which would always remain as they are, pro 
vided the climate underwent no radical change and they ver 
not interfered with by man. It is for the vegetable physiologist 
to say why this fineness of the soil is unfavorable to the gro 
of trees; it is for the geologist and physical geographer to set 
forth the facts which they may observe within the line of their 
own professional work. ; 
From the point of view here established it is easy to expla 
E SERE 
EA AEE E E non A NE ANA EES 
