68 BULLETIN UNITED STATES GEOLOGICAL SUKVEY. [ToZ.TI, 



therefore wherever there is rain enough, forest must needs cover the ground. At 

 least there are some curious exceptions to such a general rule — exceptions both ways. 

 In the Sierra Nevada we are confronted with a stately forest along with a scanty rain- 

 fall, with rain only in the three winter months. All summer long, under those lofty 

 trees, if you stir wji the soil you may be choked with dust. On the other hand, the 

 prairies of Iowa and Illinois, which form deep bays or great islands in our own forest- 

 region, are siJread under skies which drop more rain than probably ever falls on the 

 slopes of the Sierra Nevada, and give it at all seasons. Under the lesser and brief 

 rams we have the loftiest trees we know; under the more copious and well-dispersed 

 rain, we have prairies, without forests at all. 



There is little more to say about the first part of this paradox, and I have not much 

 to say about the other. The cause or origin of our prairies — of the uuwooded dis- 

 tricts this side of the Mississippi and Missouri — has been much discussed, and a whole 

 hour would be needed to give a fair accouut of the different views taken upon this 

 knotty question. The only settled thing about it is that the prairies are not directly 

 owing to a deficiency of rain. That the rain-charts settle, as Professor Whitney well 

 insists. 



The prairies which indent or are inclosed in our Atlantic forest-region, and the 

 plains beyond this region, are different things. But, as the one borders — and in Iowa 

 and Nebraska passes into — the other, it may be supposed that common causes have 

 influenced both together, perhaps more than Professor Whitney allows. 



He thinks that the extreme fineness and depth of the usual prairie soil will account 

 for the absence of trees ; and Mr. Lesquereux equally explains it by the nature of the 

 soil, in a different way. These, and other excellent observers, scout the idea that im- 

 memorial burnings, in autumn and spring, have had any effect. Professor Shaler, 

 from his observations in the border land of Kentucky, thinks that they haj^e — ^that 

 there are indications there of comparatively recent conversion of oak-openings into 

 prairie, and now — since the burnings are over — of the reconversion of prairie into 

 woodland. 



I am disposed, on general considerations, to think that the line of demarcation be- 

 tween our woods and our plains is not where it was drawn by nature. Here, when 

 no physical barrier is interposed between the ground that receives rain enough for 

 forests, and that which receives too little, there must be a debatable border, where 

 comparatively slight causes will turn the scale either way. Difference in soil and 

 difference in exposure will here tell decisively. And along this border, annual burn- 

 ings — for the purpose of increasing and improving buffalo feed — practiced for hun- 

 dreds of years by our nomade predecessors, may have had a very marked effect. I 

 suspect that the irregular border line may have in this way been rendered more irregu- 

 lar, and have been carried farther eastward wherever nature of soil or circumstances 

 of exposure predisposed to it. 



It does not follow that trees would re-occupy the land when the operation that de- 

 stroyed them, or kept them down, ceased. The established turf or other occupation 

 of the soil, and the sweeping winds, might prevent that. The difficulty of reforesting 

 bleak New England coasts, which were originally well wooded, is well known. It ia 

 equally but probably not more difficult to establish forest on an Iowa prairie, witk 

 proper selection of trees. 



The difference in the composition of the Atlantic and Pacific forests is not less 

 marked than that of the climate and geographical configuration to which the two are 

 respectively adapted. 



With some very notable exceptions the forests of the whole northern hemisphere in 

 the temperate zone (those that we are concerned with) are mainly made up of the 

 same or similar kinds. Not of the same species ; for rarely do identical trees occur in 

 any two or more widely separated regions. But all round the world in our zone the 

 woods contain Pines and Firs and Larches, Cypresses and Junipers, Oaks and Birches, 

 Willows and Poplars, Maples and Ashes, and the like. Yet with all these family like- 



