343 Geology and Botany of Northern Pacific Railroad. 



south. Here, between the eastern drift and that from the 

 Rocky Mountains, the soil is formed entirely by the decomposi- 

 tion of the underlying rocks ; and wherever these are shales and 

 calcareous sandstones, as they are throughout most of the Creta- 

 ceous formation, there are no outcropping ledges of rock, the 

 country is smooth, and stone of all kinds is scarce. 



This belt, which runs from the Mexican to the Canadian line, 

 is prairie because of the dryness of the climate, and not on ac- 

 count of the geological substructure ; for, between the " Cross- 

 timbers" and the Raton Mountains, with a great variety of geol- 

 ogy and topography, there are no trees except along the water- 

 courses ; which, fed by the melting of the snow on the Rocky 

 Mountains, are perennial, and supply constantly the amount of 

 moisture that is a necessity for tree growth. The peculiar tine- 

 ness of the soil of the northern portion of this belt has been sup- 

 posed to have something to do with the prevalence of grass and 

 the absence of trees ; since in Illinois and Wisconsin, along the 

 border line between the forest area and the prairie, the levels 

 where the soil is fine are .grass-covered, while the swells and ridges, 

 rocky or gravelly, carry trees ; but as I have shown elsewhere, 

 these local peculiarities of the soil, favoring, the first grass and 

 the second trees, have simply caused the interlocking of prairie 

 and forest along the debatable line. 



Further west, with every kind of soil, geological structure and 

 topography, there are no trees, but everywhere grass ; while east 

 of the Mississippi and beyond the battle-ground between the two 

 forms of vegetation, all kinds of topography, soil and geological 

 substructure are covered with forest. No one who has traversed 

 the continent, as I have done, along several parallels of latitude, 

 and has studied the relations of vegetation to soil and geological 

 structure, will fail to find conclusive evidence that the influence 

 which has determined the kind and quantity of vegetation in the 

 varied topographic and climatic districts of the West, is the 

 rainfall. 



The valley of the Little Missouri is deeply cut in a table-land 

 composed of the Laramie coal-measures, of which 200 or 300 feet 

 are exposed in the cliffs, with several seams of coal. Thousands 

 of silicified tree-trunks lie scattered over the surface, and innu- 

 merable stumps are standing apparently where they grew ; but 

 no foreign material is anywhere visible. 



