bussell. I TWO TERTIARY LAVAS. 61 
with the Columbia, that the relation of the two becomes of interest.^ 
The country separating these two areas of basaltic rock has not as yet 
been geologically explored, and while it is possible that a direct con- 
nection between them may be discovered, it seems more probable that 
they are distinct and have somewhat different histories. From the evi- 
dence now in hand, it appears that the Snake River lava is in general 
much younger than the related formation to the westward, although 
its basal members seem to be of about the same age. The principal 
part of the Columbia River lava, as shown by fossil leaves contained in 
the associated sedimentary beds, is of Miocene age/' The older por- 
tion of the Snake River lava, as indicated by its relation to the Payette 
formation, etc., as stated on a previous page, is also of Miocene age. 
Much reliance should not be placed in this correlation, however, until 
it is sustained by additional evidence. Although the first-formed 
sheets of the Snake River lava are perhaps of about the same age as 
the main mass of the Columbia River lava, by far the greater part of 
it is much younger. The latest outpourings of molten rock over the 
Snake River Plains, as will be described later, occurred probably 
within recent historical times, and are perhaps not over 100 to 150 
years old. While the Columbia River lava is deeply decayed and over 
large areas changed to a soft clay-like soil having a depth of 60 feet 
or more, the lava sheets on the Snake River Plains are still fresh, and 
even in the case of the exposed portions of the older sheets show but 
slight changes. These older sheets may have been covered for a long 
time by sedimentary deposits which have since been washed away, and 
thus preserved from decay, but this explanation can not be made to 
apply to the greater part of the lava. The evidence furnished by a 
comparison of the state of preservation of the Snake River and the 
Columbia River lavas shows conclusively that the Snake River lava 
is in general much the younger of the two. 
HYPOTHESIS OF FISSURE ERUPTIONS. 
There is but little literature relating to the Snake River lava, but 
what has been published in reference to its origin (except by Lindgrcn) 
"Descriptions of the Columbia River lava may be found in the following publications: 
Geoige Gibbs, Report on the geology of the central portion of Washington Territory: Pacific 
Railroad Reports, Vol. I, 1854, pp. 473-486. 
F. von Richthofen, The natural system of volcanic rocks: Mem. Cal. Acad. Sci., Vol. I, Part 
II, 1868. 
Joseph Le Conte, Am. Jour. Sci., 3d series, Vol. VII, 1874, p 168. 
T. W. Symons, Report on an examination of the Upper Columbia River, Senate Ex. Doc. No. 186, 
Forty-seventh Congress, first session, 1882. 
I.C. Russell: A geological reconnaissance in central Washington, Bull U, S. Geol. Survey, No. 
108, 1893; A reconnaissance in southeastern Washington. Water-Supply and Irrigation I'apers U.S. 
Geol. Survey, No 4, 1897; Volcanoes of North America; The Macmillan Co.. New York, 1897; 
Geology and water resources of Nez Perce County Idaho Water-Supply and Irrigation Papers, U.S. 
Geol. Survey, Nos. 53 and 54, 1901 
bF. II. Knowlton, Bull, U. S Geo! Survey, No 108, 1893, pp. 103-104. 
