100 IOWA ACADEMY OF 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 affirmed 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 thej?- failed 

 to show or see how or why such difference avai]ed. 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: (i) 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, lurnished 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 



