96 United States Survey of the Territories. | February, 
explains the distribution of timber in slightly hilly and level 
regions like the one under consideration, as has been shown 
above. It explains the existence of prairies where the rain-fall 
may be extreme even, for the precipitation of moisture may be 
caused by extreme changes of temperature, and the changeable 
climate which may produce much rain may also produce intervals 
of great dryness. It explains why the timber areas may some- 
times correspond to geological formations, as urged by Professor 
Whitney. The soil either carrying the rain-fall away rapidly or 
retaining it; either cutting off the surface from the springs of 
water below, or drawing it up like a sponge; either rendering the 
climate more uniform by its better conducting power, or allowing 
the changes of the atmosphere to govern too perfectly the tem- 
perature of the surface of the ground. It may explain to some 
degree the fact that the grass of the prairie prevents the advance 
of timber in a hilly region by its preventing the showers entering 
and moistening the roots of larger forms of vegetable life. It 
also suggests certain means for securing the healthy and contin- 
- ued growth of groves, indicating the more favorable positions for 
them, showing methods of economizing the rain-fall in places 
where it may be scanty, etc. 
In conclusion, therefore, while acknowledging that prairie fires, 
=- the amount and distribution of rain-fall, the nature of the soil, 
the temperature and inclination of surface, that all may have more 
or less importance in explaining the origin of forests and prairies ; 
we may nevertheless be convinced that the fundamental condition 
of forest growth is a constant medium humidity of air and sou, 
Let us, therefore, while not neglecting our pluviometers look more 
carefully to our hygrometers in our study of this subject. 
oo 
THE FIELD-WORK OF THE UNITED STATES GEO- | 
LOGICAL AND GEOGRAPHICAL SURVEY OF THE 
TERRITORIES, UNDER THE DIRECTION OF PROF. 
F. V. HAYDEN, FOR THE SEASON OF 1877. 
N the completion of the survey of Colorado last year, it was de 
termined that the work of the United States Geological and Geo- — 
ee graphical Survey of the Territories, under the direction of Prof. 
-Hayden should continue northward into Wyoming and Idaho. The 
belt of country including the Pacific Railroad having been explor d 
