iSgi.] 
245 
[Wright. 
Nampa is within five miles of the western edge of this last area, 
and no basalt appears for 75 or 80 miles at least lower down the 
valley. 
The age of this lava may eventually be determined by studying 
the problems of erosion presented, but I am not prepared at pres- 
ent to express even an approximate opinion. This western centre 
of dispersion which overran Nampa seems to be separated from 
the larger area to the east ; and from the fossils which I found un 
derlying it in the vicinity of Glenn’s Ferry, there can be no ques- 
tion that the lava is either post Tertiary or late Pliocene. Among 
iny specimens, Mr. W. II. Dali of Washington has identified the 
following: Goniobasis taylori Gabb (sp.), Lithasia antiqua Gabb, 
Latia dalli White, Sphaerium idahocusis Meek, and ? S. negosum 
Meek. “These fossils characterize the ‘Idaho formation’ of Cope ; 
the rocks belong to these dimentation of Cope’s ‘Idaho Lake’ 
and are Pliocene ; very likely middle or later Pliocene.” In 
a note, Mr. Dali adds: “These rocks were synchronized at one 
time with King’s Truckee group, formerly referred (on very in- 
sufficient evidence) to the Miocene.” 
As shedding some light upon the situation, it is important to 
observe that the most of this basalt is upon the north side of Snake 
River, and that in its outflow it has pushed the Snake River 
against a long line of Tertiary beds all the way along from a 
short distance below Shoshone falls to the Oregon line, while in 
the neighborhood of Nampa, the lava flow has been thrust in as 
a wedge, driving apart for some distance the Snake and Bois6 
Rivers, and the deposits of quicksand and clay underneath the 
lava at Nampa did not take place in a lake, as I at first surmised, 
but are in all probability alluvial deposits in a river valley, when 
its drainage was suffering obstruction from the eruptions of lava 
and when (probably in connection with the glacial period) there 
was a much larger volume of water than now coming down from 
the mountains to the north and east. 
The correspondence between the conditions in the neighborhood 
of Nampa and those in Calaveras and Tuolumne counties, Cali- 
fornia, where Professor Whitney discovered human relics under 
the lava of Table Mountain, seem to me in a broad way very 
striking. There is about the same evidence of increased water 
flow ; the amount of erosion in the superincumbent lava is ap- 
proximately the same ; and the underlying fossils indicate the 
same general geological period. 
