320 The American Geologist. November, luui. 
occasionally it has layers of clay, ■ lignite, sand and gravel. It is 
nearly horizontal. The older group carries some gold and ores of 
other metals, but on decay forms a very poor soil. The younger 
is without native metals and ores but forms a deep dark soil of mar- 
velous fertility. Possibilities of artesian water inhere only in the 
younger group. The age of the prc-Tertiary group is, in part at 
least, probably Mesozoic, this being indicated by fossils found in 
certain limestones. The younger group is Tertiary, as shown by fos- 
sils found associated with some of the sedimentary beds lying be- 
tween basalt sheets. 
Prof. Russell judges that in southern and central Idaho, in the 
Glacial epoch, there were only isolated Alpine glaciers, and no gen- 
eral or continental ice-sheet. These Alpine glaciers produced one of 
the most marked of the drift phenomena of Idaho, viz. the overwash 
gravels. The great lava plain is called the "Columbia River lava. ' 
Through it protrude many isolated peaks or islands of the pre-Ter- 
tiary rocks. These are dominated, generically, stcptocs, a name de- 
rived from Steptoe butte, near Garfield, in Washington. The largest 
of these known is Eagle Creek range, or Powder River mountains, 
in Oregon, which, as stated by Lindgren, consists of bare ragged 
peaks that rise several thousand feet above the basaltic plateau, sur- 
rovmding them on all sides. This steptoe has a diameter of twenty- 
four miles. 
The Columbia River lava is the most remarkable feature of the 
geology of the region. It is almost entirely witliin the drainage 
area of the Columbia river, covering nearly the whole of Washing- 
ton and Oregon east of the crest of the Cascade mountains, and ex- 
tends into Idaho to the Coeur d'Alene and Bitterroot mountains. Its 
whole area is estimated at 200,000 or 250,000 square, miles, its great- 
est known thickness beimg 4,000 feet. Following is Prof. Russell's 
terse description : 
'"The lava was outpoured at successive intervals, embracing a 
long period of time, as is shown by the occurrence, at several hori- 
zons, of layers of sedimentary material, principally clays and sands, 
between the lava sheets. In places also the lava sheets are separated 
by layers of volcanic dust containing the silicified trunks of trees 
which grcAV on a soil formed by the decay of the underlying layer, 
thus showing that the intervals between the flows were in some 
cases a century or more in duration. The lava came through fissures 
in the earth's crust — in what are known as fissure eruptions — and 
spread widely over the land, from which it is evident that each sheet 
was spread out horizontally. The movements that have occurred in 
the layers since they cooled and hardened, and which have caused 
them to be deformed from their original horizontal position, can be 
studied in the .same way as the structure of sedimentary beds. Al- 
though the lava sheets are still essentially horizontal over broad 
areas, they frequently have gentle dips, and in certain regions are 
tilted and even sharply folded and faulted. On the eastern slope of 
