74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ment, have been shown by Gilbert (p. 366) to be no longer level. 
Some features noted in the Bighorn Mountains, which bear on the 
same point, will be discussed later. 
In a recent paper Darton (d, p. 22) states that the river valleys of 
the Great Plains are now being built up rather than deepened. 
Development of Drainage. Jaggar (a, p. 186) estimates the amount 
of Pre-Oligocene erosion at 6,000-8,000 feet. He thinks that during 
this process dome structures, which were not apparent at the initial 
surface, were gradually revealed and exercised an important influence 
upon the streams. After describing the drainage of several laccolithic 
domes he summarizes their erosion stages as follows (a, p. 276-7): 
“The cases cited show that laccolithie domes deflect regional master 
streams, and that subordinate drainage conforms strikingly to the 
relative resistance of rocks exposed. An early stage produces a dome- 
shaped hill with radial drainage. One radial stream gains advantage 
over its fellows and eats out the central portion of the dome to a soft 
stratum beneath. The outward dipping hard beds are undermined 
and drainage formerly radial outward becomes radial inward; a former 
mountain becomes a quaquaversal basin, inclosed by a horseshoe 
ridge. Recession of this ridge and continued erosion on the soft bed 
uncovers an arch of harder rock. Monoclinal shifting on the soft bed 
becomes easier than deep cutting into the dome, so the flanking beds 
are eroded down and a new radial drainage forms with two pronounced 
encircling streams.... The alternations from domical mountain to 
horseshoe-shaped ridge will continue under the erosive action of 
changing subordinate drainage until the porphyry is reached. Here 
monoclinal shifting is no longer possible, owing to lack of monoclinal 
structure.” 
In his discussion of Crook Mountain Jaggar (a, p. 272), expresses 
his strong inclination to the belief that Whitewood Creek was super- 
posed upon the mountain from a position stratigraphically higher and 
unaffected by doming, and cites as evidence the apparent deflection 
of the stream from its course by the mountain. That monoclinal 
shifting of streams on the dome in question has had much to do with 
the present arrangement of drainage in that region the writer has 
little doubt. In fact it seems highly probable that Sandy Creek and 
the stream which now drains Crook valley originated as radial streams 
on Crook Mountain, before the Minnekahta limestone capping had 
been removed, and by shifting downward along the dip of the hard 
limestone rapidly cut their way in the soft Spearfish Red Beds. Prob- 
ably, too, what may be called the lower Whitewood Creek and the 
