122 The American Geologist. February, 1903 
higlier than in Idaho, and in certain locaHties meets the cold 
timber-Hne and the mountains are bare of trees from base to 
summit. The dry timber-Hne decreases in elevation when 
traced from arid to humid regions. In the central part of the 
continental basin of North America, it defines the border of 
the treeless portion of the Great Plateaus and the prairie plains, 
and at the north coincides with the southern limit of the sub- 
arctic forest. On the borders of the treeless plateaus and the 
prairie plains the position of the margin of tl'\e encircling for- 
est is determined mainly by lack of moisture, but is varied lo- 
cally by soil conditions, hot winds, forest fires etc., in the same 
manner that the lower limit of tree growth on the mountains of 
arid region is regulated. 
When the humidity is sufificient for the growth of trees, as 
for example on the mountains of New England, the dry tim- 
ber-line disappears. An arid region may be bordered at a low- 
er elevation, by a region with sufficient humidity to permit 
trees to grow, and may then be bordered both above and below 
by the dry timber-line, as is the case in Idaho. Where an arid 
region reaches sea level as in Arizona, southern California, 
and the west coast of Mexico, etc., there is no forest below the 
arid belt, and in certain localities the dry timber-line meets 
the cold timber-line, and the mountains are bare of trees from 
sea level to their summits. 
There is also a third general cause which draws a limit to 
timber growth, namely, excessive humidity, as for example 
.on the borders of swamps, the margin of lakes etc., which may 
perhaps be termed the "wet timber-line." 
REVIEW OF RECENT GEOLOGICAL 
LITERATURE. 
Report of tlic state geologist on the mineral industries and geology of 
eertain areas of Vermont. Third of this series. 1901-1902. Geo. H. 
Perkins. 
Besides sketches of two tarly geologists of Vermont (Thompson 
iin Wing) Prof. Perkins lias presented in this excellent report, with 
many vivid half-tone plates of natural geologic sc5ne;s. more full and 
accurate descriptions than have ever before been published, of some of 
the important points in the geology of Vermont. There is a special 
