270 TERENCE T. QUIRKE 
there is also a soft, sandy limestone of loose texture carrying gravel. 
Similar pebbles and residual masses of limestone were found out 
of place on the lower slopes of the south side of Indian Knob, but 
the pebbly limestone could not be traced back to the rock still in 
place. Also, on the south side of North Mountain a few scattered 
pebbles of the same general description were found, but again none 
could be found in place. The pebbles are of quartz and igneous 
rocks, with many limestone pebbles, some of which are pale green 
in color. It is conceivable that some of these pebbles were carried 
into the region where limestone and fine material were being 
deposited by cakes of ice or in the roots of floating trees. But at 
the southwest end of the mountains the abundance of the gravel 
leads to the supposition that at that place there may have been 
the debouchure of a river emptying into a lake. Furthermore the 
lower hard ledge does not continue distinctly in the southwest end 
of the mountains, whereas in the northern part it is almost solid 
limestone. From the fact that the limestone is more pure in the 
north end of the deposits it is argued that that end was farthest 
from the entrance of the river. Another item to support the sup- 
position that the material was brought by rivers from the south is 
the fact that the gravels in White Butte are much coarser than any 
which have been found roo miles farther north in the Killdeer 
Mountains. If it is argued that these deposits were laid down in 
small lakes connected by a single river, it should be remembered 
that gravel could not have been carried through one lake and on to 
deposition in the next. And as there is gravel in the deposit in 
South Dakota, again 50 miles farther north in White Butte, and 
again 100 miles farther north in the Killdeer Mountains, it must be 
supposed that the gravels were already distributed along the course 
of the river by some earlier and uninterrupted stream. This 
accords with Douglass’ theory that the Oligocene deposits are in 
the channel of an earlier river. 
The alternate hypothesis is that only the lower beds represent 
the work of a river, and that the upper beds, which are free of gravel 
deposits, were formed in a lake which overlapped the river deposits. 
The scattered outcrops of Oligocene deposits are explained better 
as the remnants of an almost eroded formation than as deposits 
