russell.] PETROLEUM. 69 
to the east of Juniper and Mann lakes, which have a gentle surface 
slope on one side and a steep escarpment on the other. 
A characteristic example of a broad downward fold or syncline may 
he seen from the summit of Stein Mountain on looking southwest 
over the region drained by Donner and Blitzen River. The longer 
axis of this basin trends in a north-south direction, and is inclined 
downward or pitches northward. 
Accompanying the deformation of rocks, breaks sometimes occur, 
and the edges of the broken layers on the opposite sides of a fracture 
are upheaved or depressed with reference to each other. In this and 
yet other ways what are termed faults are produced. 
Breaks in the earth's crust of the nature just referred to, when they 
affect the relief of the surface, produce bold escarpments. The strata 
frequently dip away at gentle angles, thus producing what it is con- 
venient to term monoclinal ridges or mountains — that is, elevations 
with steep descents on one side and gentle slopes, corresponding 
closely with the inclinatif ns of the component beds, on the other. 
Examples of such monoclinal mounta ns are furnished, as seems evi- 
dent from such studies of them as it has been practicable to make, by 
Stein Mountain and Mahogany Mountain, in Oregon. 
MINERAL RESOURCES. 
PETROLEUM. 
Search for petroleum has been carried on in the valley of Snake 
River in southwestern Idaho and adjacent portions of Oregon, but 
these attempts have proved failures so far as the discovery of the 
material sought is concerned. In all of the instances referred to that 
have come under my notice, it is evident that little if any study of the 
geological conditions was made previous to drilling, and little if 
any appreciation of the conditions governing the accumulation 
and storage of petroleum seems to have been entertained by the 
persons who "promoted" the various oil ventures. As much time 
and money has been spent in the region mentioned in fruitless attempts 
to discover petroleum, and as additional efforts will no doubt be made 
in the same direction, more conservative methods may perhaps be 
favored if a plain statement of the general principles pertaining to 
the occurrence of petroleum, and of gas with which it is commonly 
associated, be laid before the reader. 
CONDITIONS FAVORING THE NATURAL STORAGE OF PETROLEUM AND 
ROCK GAS. 
The geologist in searching for oil and gas pools, as subterranean 
accumulations of petroleum and natural or rock gas are commonly 
termed, must of necessity be guided mainly by the physical proper- 
ties of these substances; by the occurrence of water, with which they 
