


ParrI.Sxor.i.§ 3] | FISSURE. ERUPTIONS. On 
roduced either by the lava rolling over a plain or lake-bottom, or 
2 the complete effacement of an original undulating contour of 
the ground under hundreds of feet of lava in successive sheets. 
The lava rolling up to the base of the mountains has followed 
the sinuosities of their margin, as the waters of a lake follow 
its promontories and bays. The author crossed the Snake River 
plain in 1879, and likewise rode for many miles along its northern 
edge. He found the surface to be everywhere marked with low 
hummocks or ridges of bare black basalt, the surfaces of which 








































































































































































































































































































































































































































































































































‘ 
ETP AMON NY of TP NS Ew ue 
-. Fig. 63—VIEw OF THE GREAT BAsALT PLAIN OF THE SNAKE River, IDAHO, 
WITH RECENT CONES. 
exhibited a reticulated pavement cf the ends of columns. In some 
places, there was a perceptible tendency in these ridges to range 
themselves in one general north-easterly direction, when they might 
be likened to a series of long, low waves or ground-swells. In 
many instances the crest of each ridge had cracked open into a long 
fissure which presented along its walls a series of tolerably symme- 
trical columns (Fig. 63). That these ridges were original undulations 
of the lava, and had not been produced by erosion was indicated by 
the fact that the columns were perpendicular to them and changed in 
direction according to the form of the ground which was the original 
cooling surface of the lava. Though the basalt was sometimes 
vesicular, no layers of slag or scorie were anywhere observed, nor 
did the surfaces of the ridges exhibit any specially scoriform 
character. 
_ here are no visible cones whence this enormous flood of basalt 
- could have flowed. It probably escaped from many fissures still 
concealed under the sheets which issued from them. That it was 
not the result of one sudden outpouring of rock is shown by the 
S 
