MAltVIKE.] 



GEOLOGY— CRETACEOUS OF MIDDLE PARK. 155 



render them easily recognizable. These sandstones, therefore, naturally 

 occur near to, and dipping at angles of from 10^ to 50° away from, the 

 ridges of archgeau rocks surrounding the Cretaceous areas. This group 

 appears to vary from about 350 or 400 feet, near the Hot Springs, to, per- 

 haps, 1,200 feet, near the head-waters of the Muddy. 



At the summit of the formation there is also a series of sandstones. 

 Here, however, the predomiueut colors are rusty-brown or yellow, and 

 while shaly sandstones occur, the more massive beds are soft and 

 friable in nature, and often concretionary. From only about 700 feet at 

 the east, these may become 1,000 feet thick farther west. Between these 

 two series of sandstones, and all through the middle of the formation, 

 embracing an average thickness of, perhaps, 200 feet, dark argillaceous 

 slaty shales prevail. These naturally contain arenaceous beds here and 

 there, which sometimes become quite numerous, more especially toward 

 the top. ISTo well-defined division of these middle beds was observed. 

 In the northwest portion of the park, however, and about one-quarter 

 or one-fifth of the way up from the base of the shales, is a horizon 

 at which calcareous beds tend to occur. These usually appear as 

 thin limestones, consisting sometimes of two or three more prominent 

 beds, with several minor ones, the thickest reaching, perhaps, not much 

 more than 15 feet. Though inclined to tabular, or irregularly thin-bed- 

 ded, the limestone is sufficiently harder than the adjacent shales to ap- 

 pear, when thick enough, as a hog-back-like ridge above the surface. 

 It is brownish compact saccharoidal, and often almost wholly made up 

 of Cretaceous fossils, the i3rincipal one of which resembles closely Ino- 

 ceramus acutirostris, a fossil of the Cretaceous No. 2, of the Nebraska 

 section of Meek and Hayden. These limestones bear constantly a 

 characteristic odor of petroleum. 



The usual Cretaceous fossils are found here and there through the 

 middle shales, and appear quite numerous in the upper friable sand- 

 stones. At one point in the latter, fossils, probably the Inoceramus har- 

 abini (Morton) of the Cretaceous Xo. 5 of Nebraska, were found. 



There thus appears to be a very striking resemblance between the 

 general arrangement of these groups and the five divisions of the Cre- 

 taceous, while the lower and upper sandstones present lithological char- 

 acters remarkably similar to those of No. 1 and No. 5, respectively, of 

 Hayden's divisions of the Cretaceous as present east of the mountains. 

 The true equivalent of No. 3 may not have been recognized, and though 

 the Inoceramus acutirostris (M. and H.) of the thin limestones which 

 occur in the lower middle slates may indicate it as belonging to No. 2, 

 yet this horizon of calcareous sediment would seem, on other grounds, 

 to be the real representative of No. 3, and is so given on sections 2 and 

 3", Plate HI. A complete examination of the fossils is needed to estab- 

 lish the true correlations. All these softer portions of the formation 

 occupy the valleys, and are mostly covered with subsequent beds, ren- 

 dering their characters not readily studied, and not always clear. The 

 representatives of Nos. 2 and 4 are much more strikingly argillaceous 

 than along the east base of the range, and may be undistiuguishable 

 from one another unless the beds assumed as No. 3 occur distinctly 

 enough to separate them. 



Though no fossils were observed in the lower siliceous sandstone, its 

 relations to the beds above, as well as its lithological character, show 

 it to be identical with \he Cretaceous No. 1, east of the range. Near 

 the entrance of the park, and on the Upper Muddy River, tliere was a 

 much greater thickness of the redder and softer beds lying between the 

 more characteristic quartzites of No. 1 and the archa3an rocks below, 



