Morse on Living Brachiopods. — Schuchert. 121 
determined. The plates accompanying this memoir were 
drawn by the author in his usual clear and artistic manner. 
They represent just what is intended to be shown, and are ev- 
idently depictions of natural objects." (Science, Dec. 5-1902, 
pp. 901-902). 
TIMBER LINES.* 
By Israel C. Russell, Ann Arbor, Mich. 
"Timber-line" as commonly defined, is the upper limit of 
arboreal vegetation on mountains. Its position is determined 
mainly by the occurrence of a mean annual temperature of 
about 32 degrees Fahrenheit, but locally its elevation is regu- 
lated by soil conditions, and by differences between various 
localities in snow-fall, severity of winter storms, exposure to 
the sun, etc. It may with propriety be termed the "cold timber- 
line." Above it on high mountains, there is commonly a re- 
gion occupied by alpine flowers, and still higher a region of 
perpetual snow. When traced from warm to colder regions, 
or in general from equatorial toward polar regions, it becomes 
lower and lower ; in North America it descends nearly to sea 
level in Alaska and northern Canada, where it defines the nor- 
thern limit of the sub-arctic forest and becomes the "contin- 
ental timber-line" to the northi of which lie the barren grounds 
and tundras, which correspond to the zone of alpine flowers 
on loftv mountains in temperate latitudes. 
On some of the mountain ranges of the arid portion of the 
United States, there is a lower limit of tree growth, the posi- 
tion of which is determined in the main, by insufficient moisture, 
and locally by soil conditions, including the presence of alkali, 
hot winds, forest fires, exposure to the sun etc. This may be 
termed the "dry timber-line." Below it are treeless, grass- 
covered plains and valleys. On the mountains of central Ida- 
ho, the cold timber-line is sharply drawn at an elevation of 
about 10,000 feet, while the dry timber-line, equally well de- 
fined, has an elevation of about 7.000 feet ; between the two 
there is a belt of forest trees which encircles the mountains. 
In southeastern Oregon, Nevada, southern California, etc., 
where the climate is excessively arid, the dry timber-line is 
•Abstract of a paper read at the late meeting, G. S. A., at Washington, D. C. 
