INTRODUCTION. 13 
The general country rock is Kaibab limestone in horizontal layers 
forming the plateau, surmounted by lavas over extensive areas near 
the mountain. The bedrock is covered by a thin sdil, largely formed 
in place. The soil over the limestones is porous, while that over the 
lavas has much clay and holds water. There is no swampy ground and 
therefore no conservation of moisture from year to year. Consequently 
variations in moisture-supply are quickly felt by the trees. The pine 
forest is remarkable for the absence of other kinds of vegetation. It 
covers all parts of the plateau from about 5,000 feet in elevation to 
about 9,000. At the lower edge of the pine forest a belt of cedars, 
smaller than the pines and round in shape and with dark-green, thick 
foliage, makes an attractive landscape. 
Climate and seasonal conditions.—The climate follows naturally 
from the latitude and altitude and the distance from the ocean. In 
the winters there may be from 1 to 6 feet of snow on the ground at one 
time. The storms are of the characteristic temperate-zone cyclonic 
types, but on account of the altitude the preliminary south or east 
winds are rarely observed. Storms come from the Pacific coast and 
rain occurs about a day later than in southern California. Spring and 
autumn are the dry seasons, and the warmest time of year is usually 
in June, just before the summer rains begin. The summer rains occur 
in July and August and often come in “spells” that last a week or two, 
with thunderstorms in the afternoons or at night, followed by clear 
mornings. Unlike the winter storms, the summer rains are local and 
apt to be torrential in character, with heavy run-off. 
Meteorological records in northern Arizona are necessarily meager, 
yet not so deficient as might be expected. The country was first 
settled in the ‘‘fifties,’’ when gold was discovered in Arizona as well 
as in California, and lines of travel were established from Santa Fe 
westward across the plateau. The “‘blazings’’ on the pine trees 
marking the earlier roads are still to be distinguished. Soon after the 
opening of the country the government located military camps at 
various places, and from that time records of rainfall and temperature 
were kept. The record at Whipple Barracks, near Prescott, begun in 
1867, has been continued at Prescott to the present time. It is the 
longest consecutive record in the pine forest and is therefore used below. 
The extreme range in temperature observed in Flagstaff is from 
about 20° F. below zero to about 100° F. above. But the town is ina 
peculiarly sheltered position and exhibits much lower night extremes 
than the “mesas”’ 200 to 400 feet above it. I have observed a differ- 
ence of 26° F. between the top and bottom of the hill west of town at 
sunrise on a winter morning. During the early years of the Lowell 
Observatory, which is located on the mesa 350 feet above the town, 
the lower minima were about 5° F. These figures show the conditions 
to which the trees are subjected. 
