r 
252 ELDRIDGE. 
features just noticed again occur, in a minor degree and in 
a manner not at first liable to attract attention, in the rela¬ 
tions between the Dakota and underlying beds nearer the 
middle of the area, where the beds of the younger formation 
lie across the edges of those of the older. 
North of the central portion of the area of non-conformity 
and south of Ralston creek for the distance of about two 
miles the topographical and geological features are somewhat 
complicated by the presence of intrusive masses ; they are, 
however, still sufficiently clear to permit interpretation, and 
with the others in the south and center of the area and in 
the remainder of the tract to the north form one complete 
whole. 
Detailed Geology. 
The Formations and their Relations . 
The Archsean .—This is but slightly involved in the special 
geological history of the region. It formed an uneven floor 
for the deposition of the Trias, across the truncated edges of 
which the latter formation was deposited. 
The Trias .—In their strike and dip the beds of both mem¬ 
bers of the Trias are conformable inter se. Their strike 
follows approximately the line of the Archaean foot-hills, 
and their dip is to the east and varies between 35° and 90°, 
the shallower next the foot-hills and increasing as distance 
from them is gained. 
The lower member of this formation, the Red Beds, main¬ 
tains its usual appearance, and, with two exceptions, a nearly 
constant thickness over the entire area under consideration. 
The two variations in thickness are found, the one near the 
southern extent of the tract, the other for a mile and a half 
on either side of Clear creek. The former is of no particular 
interest in the present discussion. The latter is attributable 
to two causes : one, non-deposition at the base, due to a rise 
in the Archsean floor and a consequent shallowing of the 
sea at this point, the beds of the deeper water abutting against 
this rise; the other, the disappearance from the top of the 
