FLORA OF THE CASCADES 
147 
portion of the mass. The rough surfaces of the blocks showed 
scarcely a sign of weathering, and vast areas were almost ut- 
terly destitute of vegetation; only very rarely could be seen a 
low stunted specimen of Finns contorta. But there are older 
flows also, as at least three different periods of volcanic activity 
are represented. The more recent flows have followed the de- 
pressions in the older, islanding a number of elevations which 
are fairly well covered with vegetation. The width of this lava 
field where the road crosses it is, perhaps, between four and 
five miles. At its e*astern edge, at an elevation of about 1500 
meters, is a small canyon with abundant moisture where sev- 
eral Hudsonian species were found, among them Phyllodoce 
cmpetriformis and Valeriana sitchensis. 
The flora of the east slope of the Cascades may be regarded 
as beginning at the eastern edge of these lava fields, which also 
form the eastern edge of the divide. Within a distance of three 
or four miles we descend into the Arid Transition. For thirty 
miles we have been ascending very gradually through a meso- 
phytic flora from pure Transition to high Canadian, and the 
change, as we drop within an hour into the Arid Transition, is 
most striking. This seems all the more remarkable from the 
fact that there is no steep descent. In the eight or ten miles 
we have covered (somewhat less than this in a straight line) 
in passing from the summit into the Arid Transition zone, we 
have descended scarcely more than 300 meters,' and beyond this 
there is little general eastward slope. McKenzie Pass is, of 
course, one of the lowest points in the great Cascade divide, yet 
the drop to the general level is nowhere as great as we might 
expect to find, for the whole of central Oregon is a plateau reg- 
ion from 1000 to nearly 2000 meters in elevation. 
Our road follows the edge of the lava for some distance and 
Finns contorta, Abies nobilis and Tsnga mertensiana continue 
to be the dominant conifers, but soon we come upon scattered 
specimens of yellow pine, Finns ponderosa, and Fseudotsnga 
mncronata appears and suddenly becomes abundant ; it seems to 
form, however, a remarkably narrow zone for a species ordi- 
narily of such wide altitudinal range. With thei increase of the 
yellow pine the other conifers thin out, though maintaining a 
sporadic distribution for two or three miles; finally they quite 
disappear, and for five or six miles we ride through a practically 
pure growth of yellow pine. 
