russell.] STRUCTURE. 67 
Both the basalt and the rhyolite which form such extensive outflows 
in Idaho and Oregon came to the surface through openings, as fissures, 
for example; and below the present surface it is to be expected that 
there are many fissures in which molten rock has cooled so as to form 
what are termed dikes. A few dikes were observed in the bold 
eastern face of Stein Mountain, and indicate what may be present in 
other localities. The dikes referred to are nearly vertical, vary in 
width from 20 to 60 feet, and are horizontally columnar. As is well 
kuown, dikes have frequently led to the hardening of the walls of the 
fissures they occupy, and for this and other reasons their presence in 
an artesian basiu might introduce serious difficulties in the wa\^ of 
obtaining flowing wells. If in drilling a well the locality chosen should 
chance to be directly above a dike, it is probable that the well would 
be a failure, even if the difficulty of penetrating the usually compact 
and hard rock of the dike could be overcome. Where the country 
rock is concealed beneath deep sheets of soil, alluvial deposits, etc., 
it is f requently impossible to detect the presence of a dike in the rocks 
beneath the surface covering, unless they are encountered in drilling 
or making other excavations. So far as can how be judged, however, 
the dikes in the artesian basins described in this report are not numer- 
ous, and the chances of striking them in drilling are small. A greater 
danger lies in the fact that they may cut a water-bearing stratum, so 
as to prevent the percolation of water through it. 
STRUCTURE. 
By geological structure is understood, in part, the positions occu- 
pied by stratified or bedded rocks in the earth's crust. For example, 
sandstone, shale, etc., originally laid down in horizontal sheets, are 
now found to be inclined, folded, and in many instances occupy a 
vertical position. 
The rocks, both sedimentary and volcanic, which underlie south- 
western Idaho and southeastern Oregon, were at the time of their 
formation spread out in nearly horizontal layers, but subsequently 
they have been depressed in one locality and raised in another, and 
in certain instances crushed together so as to form great upward and 
downward folds. In other instances they have been broken along 
nearly vertical planes, and the portion of the strata on one side of 
the break upraised or depressed in reference to the corresponding 
strata on the opposite side; that is, have been faulted. Examples of 
the gentle tilting of previously horizontal sheets of rocks throughout 
great areas are furnished in the broad Snake River Plains between 
Glenns Ferry and Owyhee. North of Snake River the rocks are 
now gently inclined upward to the north, but south of the river the 
same beds rise when followed southward, and in the Owyhee Moun- 
tains are more than a thousand feet above the position the same 
strata occupy beneath Snake River. This uptilting of stratified beds 
is well displayed in the hills near where Snake River first crosses the 
