244 ANNUAL OF SCIENTIFIC DISCOVERT. 



as "oak openings" indicate the transition from the densely 

 wooded region to the treeless plains. The trees stand as an arti- 

 ficial park, shading a greensward devoid of underbrush, so that 

 the traveller may ride or drive in any direction. This character- 

 istic he had noticed almost continuously from Green Bay to the 

 Tvestern borders of Arkansas. The trees appear dwarfed and 

 sickly, the extremity of the limbs is often dead, while the main 

 body is covered with foliage, and the trunks when felled are found 

 to be more or less decayed. The change in the character of the 

 grasses and herbaceous plants is more marked even than in the 

 trees. The cactus-plant forms one of the most noticeable plants 

 of the prairie. These plants are the pioneers of a marked change 

 in a vegetation which finds its full development still further in the 

 interior of the continent, and may be regarded as the unerring 

 index of a change in the condition of humidity in the atmosphere, 

 rather than in the mechanical texture of the soil. 



The fourth division of the country extends between the Missouri 

 River and the base of the Rocky Mountains. As the traveller ad- 

 vances from east to west, he l)Co;ins to find increasino^ sio^ns of 

 dryness in the atmosphere. The rain-fall is insufficient for the 

 cultivation of the crops, and the diurnal changes of temperature 

 are too abrupt to permit the growing and maturing of the sub- 

 tropical plant. The soil is sandy and jDorous, the surface in pla- 

 ces is covered with incrustations of soda and gypsum, and the 

 streams are rendered unpalatable by reason of the solution of 

 these salts in their waters. The vegetation indicates a similar 

 change of climatic conditions. 



The Rocky Mountains form a well-marked division in the cli- 

 matology of the United States, as well as in the fertility of soil 

 and the distribution of plants. While on the eastern slope we 

 have immense grassy plains, large accumulations of detrital ma- 

 terials, and a gently rolling surface, on the western slope we have 

 large tracts of sandy wastes, of rocky surfaces bare of covering 

 and intersected by numerous and deep caiions, so intricate as to 

 bewilder and impede the explorer. The great basin and the Col- 

 orado desert occupy the region between the Rocky Mountains and 

 the Sierra Nevada from the head of the Gulf of California as far 

 north as latitude 42°, and in many respects present physical as- 

 pects not elsewhere recognized in North America. 



It is a noticeable fact, that while on the Atlantic slope the pre- 

 cipitation of moisture is pretty equally distributed over the four 

 seasons, the tendency to unequal precipitation, comparing spring 

 and summer with autumn and winter, begins to manifest itself on 

 the prairies, and as we enter the plains it becomes still more 

 marked, — the fall, and especially the winter, being the dry season. 

 Having shown, by a series of elaborate calculations, the compara- 

 tive rain-fall in the difi'erent divisions of the continent in each season 

 of the year, he concluded by expressing his belief that a study of 

 the physical features of this country, in connection with the pre- 

 vailing wind, and the consequent distribution of moisture, and 

 also in connection with the lines of equal temperature, will justify 

 us in drawing the following conclusions: 1. That these great 



