184 GEOLOGY OF EASTERN WISCONSIN". 



Distribution. On the accompanying map of vegetation, the dis- 

 trict covered by each of these classes is delineated, with as much of 

 detail and accuracy as was compatible with the demand of other de- 

 partments of the survey, and with the rapidity necessitated by the 

 large area examined. As that presents their distribution far more 

 vividly and accurately than any verbal description, I need only add 

 here a few supplementary remarks. 



It will be seen by consulting the map, that the prairies are almost 

 exclusively confined to the southwestern portion of the district, or 

 chiefly to the Mississippi drainage system, though this fact perhaps 

 has no special significance. The surface of a portion of these prairies 

 is level, and bears evidence of having formerly been a lake bottom, 

 while that of others is elevated and undulating, and bears no evidence 

 of having been submerged since the retreat of the glacier. The areas 

 of prairie and forest are so intermingled as to forbid any topogra- 

 phical distinction between them, and to negative any explanation of 

 their origin that is dependent on surface features. It is not proposed 

 to enter here upon the much discussed question of the origin of prai- 

 ries, but simply to remark that the tenor of the facts in this region, 

 bearing upon the question, supports the general views so ably pres- 

 ented by Profs. Dana, dewberry, Guyot and others, and at the same 

 time harmonizes with the observations of Prof. Whitney, to the ex- 

 tent that the nature of the soil is the most essential primary agency. 

 It is not however because the soil is incapable of supporting trees, for 

 when planted upon the prairies they flourish luxuriantly, and when 

 the soil is cultivated or shaded so that a proper degree and constant 

 supply of moisture is secured, trees are propagated from the seed with 

 facility. In the first case, in planting the trees, the superficial com- 

 pact soil, which is believed to be the real barrier to the extension of 

 the forests in itds region, is penetrated, and the roots of the tree 

 placed below it, and it is at the same time loosened, and mingled with 

 the subsoil; and in the second case, artificial stirring of the soil or 

 special conditions supply the moisture essential to the growth of ar- 

 boreous vegetation. But in its native, undisturbed condition, the fine 

 superficial soil becomes exceedingly dry at intervals during the sea- 

 son, and renders it impossible or exceedingly difficult for the young 

 seedling to maintain its existence until it can gain a foot-hold upon 

 the deeper and uniformly moist subsoil. This difficulty is increased 

 by the antagonism of the grasses that can successfully withstand these 

 variations of moisture, and by annual fires. The latter have doubt- 

 less modified the form and extent of the prairies in some degree, but 

 it is generally conceded by those who have studied the subject com- 



