ocr., 1899.] SLOPE EXPOSURE. 49 
Danyon, aear the mouth of Clear Creek (see pl. 111). The altitude of 
the bottom of the canyon at this point is 6,700 feet, which would nat- 
‘urally place it in the middle of the Canadian zone. The steep west 
side of the narrow ridge between the two creeks receives the afternoon 
sun at nearly a right angle, and is in consequence an unusually warm 
slope for the altitude. The result is that seeds of plants carried upward 
by winds and birds from the Transition zone, more than 1,000 feet 
below, have here found a favorable resting place and have grown into 
a colony of Transition zone species, among which are Abies concolor 
lowiana, Arctostaphylos patula, Kunzia tridentata, Amelanchier alnifolia, 
Rubus parviflorus, Sorbus sambucifolia, Symphoricarpos pilosus, Ribes 
viscosissimum, R. amictum, Sambucus melanocarpa, Apocynum pumilun, 
Spirea douglasi, Vaccinium (arbuscula?), Lupinus elmeri, Eriogonwn 
marifolium, Gilia aggregata, Pteris aquilina lanuginosa, and the large 
Transition zone form of Castilleja miniata. Just across the canyon, 
and in one place less than 100 feet from the lower edge of this isolated 
Transition colony and at a lower level, are species belonging to the 
Hudsonian zone—such as Pentstemon newberryi and Pulsatillu occiden- 
talis, Thus, growing close together on opposite slopes of the same 
canyon, are species characteristic of zones both above and below the 
one to which the altitude properly belongs. This case is by no means 
peculiar and is a good illustration of the simpler effects of slope exposure 
commonly shown in mountain canyons. 
Another class of cases is found on the buttes and hills. A mile 
and a half northeast of Wagon Camp is « conspicuous red cinder cone 
about 1,000 feet in height, known as Red Cone. In zone position it is 
well within the lower part of the Canadian belt where it is surrounded 
by the forest of Shasta firs, scattered trees of which push up on the 
basal slopes and on the south side reach the top. Just below the sum- 
mit are a few silver pines belonging to the same zone. The warm south- 
west slope brings up from the Transition zone below thickets of man- 
zanita, a few incense cedars, and several humbler plants. The cold 
northeast slope, although reaching an altitude of only about 6,800 feet, 
which would place it in the middle of the Canadian zone, is cold enough 
to bring down from the Hudsonian zone several characteristic plants, 
such as Polygonum newberryi, Cymopterus terebinthinus, Cycladenia 
humilis, Eriogonum polypodum, and Viola purpurea. 
On the south side of Shasta an interrupted ridge or series of buttes, 
known collectively as ‘Gray Butte,’ reaches up the mountain side from 
Red Cone, east of Wagon Camp. northward to the gap known as ‘The 
Gate.’ In a gulch or canyon on the east side of this butte the black 
alpine hemlock descends more than 1,000 feet below its usual lower 
limit. The reason is obvious. The row of buttes cuts off the warm 
afternoon sun, and seepage from melting snows keeps the soil moister 
than in more exposed places, so that the gulch meets the two require- 
ments of the alpine hemlock—a moist soil and acold atmosphere. For 
its entire length this long tongue of hemlock is flanked by Shasta firs 
21753—No. 16——7 
