8l8 G. K. GILBERT 



No fossils were found below No. 5, and our present knowl- 

 edge of the general stratigraphy of the surrounding country does 

 not warrant a definite correlation of the formations. A few 

 miles farther north the Dakota formation is exposed in a broad 

 belt, beyond which it dips under the Benton shales. 



Two Butte Creek, which in general has a comparatively open 

 valley, passes at this point through a box canyon sixty feet deep, 

 and in the walls of the canyon the three highest formations are 

 seen to arch regularly from west to east. They are also seen to 

 dip southward, so that the structure revealed by the canyon is 

 essentially part of a quaquaversal arch. Scattering outcrops of 

 various formations at the north accord with this theory, and fur- 

 ther confirmation is found in the vicinity of the butte, where 

 some of the highest ground, excepting the butte itself, is occu- 

 pied by the lowest formation. No. i. The white limestone (2), 

 which outcrops in the creek valley with southerly dip, is also 

 found at the southeastern base of the butte, where it is nearly 

 level ; and the crest of the butte consists of the lower division 

 of the red sandstone (4) which there dips to the west. By 

 combining the stratigraphic and hypsometric data it was found 

 possible to estimate at many points on the sheet the height to 

 which the upper formations had been lifted ; and with further 

 aid from observed dips, contour lines were drawn to represent the 

 figure of deformation. These contours appear in Fig. 5, where 

 a distinction has been made between the parts practically fixed 

 by the observed data and other parts interpolated with a free 

 hand. So many minor inflections were found in the district 

 exposed to direct study that we may suppose the actual contours 

 to be much less regular than those supplied for the regions cov- 

 ered by the Neocene sands. At several points there are local 

 flexures, giving dips of fifteen or twenty degrees, and it is prob- 

 able that a fault traverses the western slope of the butte. 



The formation thus determined has a basal breadth in any 

 direction of from five to five and a half miles. Its central height 

 is from 1 000 to 1200 feet, the smaller estimate being derived 

 from the northwestern slope, the greater from the eastern 



