BASALTIC AND IGNEOUS ROCKS. 645 
in a wall twenty feet high and twelve wide, and the same wall may 
be traced into the crater following a north-northeast course. The 
basalt was a compact brownish-black rock, wholly uncrystalline and 
imperfectly columnar in structure. It resembles the basalt of Astoria 
and other parts of the Columbia. 
It is remarkable that the volcanic material of Swalalahos should be 
confined within three miles of the mountain on the northwest side. 
We know nothing with reference to other volcanic peaks in the 
Coast Range. From the descriptions received, it is probable that 
Mount Olympus is of the same character; but this should be received 
as a mere conjecture on imperfect evidence. 
Basaltic Rocks and Dtkes of the Columtia and Willammet.—T he 
remarks on a preceding page, have made it apparent that basaltic 
rocks or lavas extend widely over the territory of Oregon, occurring 
along the Columbia to the sources of the Snake River, over the plains 
south of Okanagan, about St. Helen’s, and at short intervals through 
the Willammet Valley and the country south. ‘These rocks in some 
instances form the surface of extensive territories, especially about 
the Upper Columbia. Again, as in the Willammet and Lower Co- 
lumbia, they occur in isolated patches or ridges through a country 
of tertiary sandstones. The hills of basalt are in some places very 
numerous and range for miles with characteristic bluff summits. 
For twelve miles north of the Boundary Range, these crested hills 
stretched along in an interrupted line; and south of the range, similar 
hills continued about fifteen miles farther, through a region of the 
tertiary sandstone—the Astoria rock. ‘The sandstone here dipped 
from twenty to twenty-five degrees to the east-northeast, or towards 
the basaltic hills. The ridges were in three or more interrupted lines, 
having the same general course. Near the southern extremity of 
the line, there was a small hillock of basalt; just back of it, there 
commenced an acclivity of sandstone, and after an ascent of two 
hundred feet, the hill declined a little, and then basalt commenced, 
which continued to the summit. Other hills beyond appeared to 
be sandstone, and others still were crested with imperfectly columnar 
basalt. Between the Elk Mountains and the Umpqua, basaltic 
ridges face others of sandstone on opposite sides of a plain not 
half a mile wide. The line of intersection of the two rocks in the 
bed of the Umpqua was concealed by the loose stones. Astoria is 
situated on a broad dike of basalt in a region of tertiary shale and 
sandstone. ‘Iwo miles to the west of Astoria, there is another dike; 
162 
