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IOWA ACADEMY OP SCIENCES. 
loess the latter. That is, to be more explicit, the loess with 
its sand and clay is a soil for cereals so poor as to raise but a 
small crop of grass, hence to furnish for sweeping fire a small 
amount of fuel, hence giving rise to less destructive fires, in 
which young trees were not universally destroyed. The drift 
on the other hand produces enormous wealth of grass, burning 
in conflagration which no seedling trees can endure; hence on 
the drift there are no trees. The presence of trees on rocky 
soils is to be explained in the same way. River bottoms fur- 
nish a special case. Here in the first case the current formed 
soil is in the nature of a sand bar, made of the coarser elements 
met with by the eroding flood. On sand bars cottonwoods and 
willows start, but not grass. The soil after a little becomes 
richer it is true, by subsiding slime, but by this time the local- 
ity is become moister than all the surrounding region; in sum- 
mer, being lower, receiving heavier dews; in winter catching 
and longer retaining a larger proportion of snow, all tending 
as check to sweeping fires. 
In conclusion, we are therefore prepared to say that all the 
students of our problems have been right, though each pre- 
sented but a partial truth. Those who afiirmed the agency of 
fire were right, but they failed to notice the fire’s selective 
operation or to explain it. Those who attributed forest dis- 
tribution to differences in soil were also right, but they failed 
to show or see how or why such difference availed. Those 
who looked back to a former geologic age were also right, 
but such failed entirely to show what the influence was which 
geologic structure has upon the problem. 
To sum up: (1) The immediate agent in the limitation and 
distribution of Iowa forests was fire. (2) The sweep of fire 
was determined by a modicum of moisture and by the presence 
of fuel upon the ground. (3) The drift being especially adapted 
to gramineous vegetation, furnished fuel in such amount as to 
prevent the development of tree- seedlings, while the loess, 
using the term in a broad sense, less suited to gramineous 
species, furnished less fuel, hence gave to tree seedlings on 
loess regions opportunity to rise. (4) Special localities, as 
swamps, alluvial flood-plains, etc., present special cases and 
require special explanations. 
As a corollary we may remark: (1) That the drift-plains of 
the state offer greatest promise to the farmer who seeks the 
cereals as his principal product. The wooded regions should 
