74 EEPOKT UNITED STATES GEOLOGICAL SURVEY. 



logical evidence spread over any extensive region has been found witbin 

 the bmits of the Jura-Trias, but we may hope that some day a lucky 

 "find" will reward patient search and set at rest all doubts regarding 

 geological age. 



CEETACEOUS. 



Of this formation we have several groups represented in our district. 

 Instead of separating the entire series of beds into five numbers, the 

 accei)tatiou of the three groups, Dakota, Colorado, and Fox Bills, is by 

 far more applicable. AH of these are developed in their regular suc- 

 cession, and are found at several localities within the district. As 

 usual, they are perfectly conformable to each other, and show thf> same 

 general characteristics that distinguish them elsewhere. The various 

 groups are more favorably developed in Professor White's district, north 

 of the river; and along the latter, only edges of his Cretaceous areas 

 reach over into my own section. East of and on the Grand Hogback 

 Eidge the greatest development of Cretaceous strata is found. In 

 regular succession the beds iollow upon each other, disturbed uniformly 

 by the force that produced the westerly dip. This at times is steep, 

 but soon, true to the hogback character, becomes gentle, and the strata 

 contiuues in a nearly normal position under the younger Tertiary beds 

 beyond. All the stratigraphical structure is so regular that it will 

 require but very little discussion, and wherever features of any particu- 

 lar interest may be observed, they will be mentioned when speaking of 

 the locality where they were found. 



Dalcota Grunj). — True to the usually observable character of this 

 group, it is here also composed of the sandstones and narrow iuter- 

 strata of dark shales, toward the upper portion of the series. A short 

 distance above the agency the Dakota sandstones cross the river, 

 coming from a more extensive area to the eastward. It forms the promi- 

 nent hill southwest of the agency, upon which Station 1 was located. 

 From there the outcrop of the sandstone runs a little east of south, op- 

 posite the great Hogback Ridge. Near the divide between the White 

 and Grand the Dakota beds form a sharp prominent hill, commanding 

 a good view over the surrounding country. The elevation of this point 

 is 9,283 feet above sea-level, about 1,800 feet higher than the valley im- 

 mediately west. Following aloug the outcrop we find that the relative 

 and absolute elevation of the sandstone decreases, and that the higher 

 points are occupied by strata belonging to the Eed-Bed series. Cre- 

 taceous, then, only flanks the higher hills, dipping steeply toward the 

 west. Farther south, opposite Station 4, the trend of the outcrop of Da- 

 kota beds is no longer a southward one, but veers off to the southeast. 

 There too the angle of the dip diminishes, and the area covered by the 

 sandstones is, in consequence, larger than farther north. Station 6 

 was located on one of the most ])rominent hills, at an altitude of 8,533 

 feet. There the dip, in conformity with the change of the line of out- 

 crop, turns slightly toward the south. In the vicinity of the Grand the 

 outcrop gradually becomes more narrow until it crosses the river. In 

 general character the sandstones of this group agree closely with the 

 l^arallel ones observed elsewhere. Varying from a yellowish white to 

 yellow and even brownish color, they present no change from the fades 

 that may be regarded as characteristic of them. Their thickness may 

 be estimated at about 1,000 feet. Among the upper members thin inter- 

 strata of dark, piirtly carbonaceous shales may be observed. They are 

 typical of this upper horizon, and may frequently aid in recognizing the 

 group. 



