SECTION FROM THE MISSISSIPPI RIVER TO POUND GAP. 39 
from such data, and especially some of those disturbing ele- 
ments which render investigation in this particular direction 
liable to error. The first, and probably the most important, 
of these is sudden and abrupt changes in the nature and rela- 
tive hardness of different parts of the same formation. In 
fact, to this cause is due, almost altogether in /z//s, and very 
largely in mountains, height above drainage itself; but it is in 
a narrower sense that it becomes a disturbing element in dis- 
cussing timber growth. The sudden cliffs and benches on 
hills and mountains are caused by difference in hardness of 
two successive strata; and a cliff of exceedingly hard rock, or 
hard, dry soil, even when near the base of a hill, will often 
be permanently drier than a bench or hill-top barometrically 
much higher above drainage. The hardness of such a cliff 
prevents the formation of detritus and the retention of water 
For this reason a softer formation or bench, easily worn away, 
and capable of forming a surface detritus which will retain 
moisture, is, so far as effect upon timber growth is concerned, 
nearer to water than a hard cliff hundreds of feet below it. 
Another disturbing element is sudden change in geological 
formation. One of the dry shales, like the Devonian and 
some of the Waverly shales, will cause as much change in 
timber growth as would be produced by the greatest height 
above drainage attainable in our mountains. Of course the 
change might be different in character, but in amount it would 
be as great. Changes of this nature, though, can usually be 
guarded against in gathering data. 
The natural difference in shade, moisture and coldness, 
between the northern and the southern faces of hills, also 
produces its effect upon the timber growth. All of these dis- 
turbing elements have been taken into consideration, and 
accounted for, as far as possible, in preparing the following 
tables. The liability to slight error, however, should be kept 
in view. 
iieupe tables, “IN. Fs,” “S..F.,” gcc, under ithe. barometric 
height at the head, mean, respectively, ‘‘ North Face,” ‘South 
Face,” &c., of the hill. 
TIM. I-——-14 209 
