70 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the streams were flowing at the level indicated by the shoulders along 
the top of the recent gorges. The fact that the result of the various 
captures has been to produce a master stream that flows northeast- 
ward, together with the fact that the master stream has been able to: 
hold its own, in spite of superposition below Crook upon anticlinal 
structures, suggests that the conditions which permitted this piracy 
resulted from a tilting of the region toward the northeast. 
A later uplift, or an acceleration of the earlier tilting, has caused 
the incision of the recent gorges and brought about the dissection and 
partial removal of the gravels. With the exception of the reversal 
of drainage at Spiegel’s Gap, little change has been made in the ad- 
justment already reached when the later movement began. 
Discussion OF THE LITERATURE. Origin of the Gravels. Newton 
and Jenney in their report (p. 44) recognize the Post-Tertiary age of 
the gravels. Darton in more recent investigations (a, p. 545) believes. 
them to be of Pleistocene age, for he finds them in occupation of 
valleys cut in the White River deposits (Oligocene). Portions of the 
gravels he recognizes as being of stream origin; but he seems to think 
that those of the Red valley and outer portions of the hills are old 
lake deposits, for he speaks of the “old shore line” of the Pleistocene 
period as “carved mainly on the limestone slopes,” while “to the east- 
ward the earlier Pleistocene plain abuts against the slopes of the 
Lakota sandstone of the hog-back ridge, excepting where it extends 
out to the plains through wide, high gaps not now occupied by water 
courses.” Crosby (p. 576) and Jaggar (a, p. 182) have also written 
with reference to the origin of the gravels. All the writers cited, 
except Crosby, express a belief in the stream origin of at least a portion 
of the gravels. 
The latter argues that the gravels in question are the residual ac- 
cumulations of thick layers of Tertiary sediments, from which the 
finer materials have been removed. Against this view the following 
facts may be advanced: (1) the distribution of the gravels is consistent 
with the arrangement of former drainage lines; (2) sections of the 
gravels often show them resting in stream-carved notches; (3) the 
slopes bordering the upper terrace level in Boulder valley appear to 
have the simple relation of valley side and aggraded floor; (4) certain 
patches of gravel, such as that of locality 16, are closely related to 
the upper valley slopes on which they lie; (5) if the gravels were 
residual accumulations, one might expect to find a larger percentage 
of cherty and concretionary forms than actually occurs. 
Quaternary Lake (?). Jenney (p. 296) and later Darton (a, p.. 
