176 REPORT UNITED STATES GEOLOGICAL SURVEY. 



FossilRidge I passed up to Spring Canon, and thence through a portion 

 of it among the foot-hills. Some minor hills that lie immediately adja- 

 cent to the foot-hills proper are formed of the hardened shales and 

 calcareous layers of the lower portion of the Colorado Group; but 

 the principal foot-hills are mostly in the form of hogbacks that were 

 produced by the upturning of the strata of the Dakota Group and the 

 Bed Beds. 



I prosecuted a labored but unsuccessful search for fossils in the strata 

 of these hogbacks, as I did, also, in those of Box Elder Canon, which is 

 a similar gorge, through these same strata, similarly upturned. From 

 their stratigraphical characteristics and relative position the Bed Beds 

 were easily recognized as those that have been generally regarded as of 

 Triassic age, and the strata of the Dakota Group were just as readily 

 identified, although no fossils were discovered. There is also a series of 

 strata between these two groups, which, being softer than the others, 

 has yielded more to disintegration. These were recognized and described 

 by the late Mr. Marvine, in his report, as Jurassic strata. lu this he 

 was probably correct, but so far as I am aware no invertebrate fossils * 

 of any kind have been found in these strata or in their equivalents east 

 of the Rocky Mountains in Colorado, except a shell found by Mr. Lakes 

 and noticed in connection with fossils found in the vicinity of Golden 

 City, on a following page. 



Returning to Fossil Ridge I made considerable collections of fossils 

 there, a part of the species being very abundant. They were obtained 

 mainly from " cannon-ball " concretions, some of which are very large 

 and which were weathered out of softer layers of sandstone. It has 

 already been mentioned that a part of these fossils are of the same spe- 

 cies that characterize the Fort Pierre Group of the Upper Missouri River 

 region. It is a well-known fact that several species of fossils are com- 

 mon alike to both the Fort Pierre and Fox Hills Groups, even in the 

 region just mentioned, and this is so especially the case in Colorado 

 that no attempt is made to separate them, and I have in my report 

 of last year ranged the equivalents of both groups under the single 

 name of Fox Hills Group, and shall do so in this report. It is neverthe- 

 less true that a greater proportion of the species found in the lower part of 

 the Fox Hills Group, as thus recognized in Colorado, are identical Avith 

 those of the Fort Pierre Group than with those of the Fox Hills Group, 

 as those two groups are recognized in the Upper Missouri River region. 

 This fact is apparent in the strata exposed in Fossil Ridge, as is shown 

 in the following list of fossils : 



Not only this fact, but also the known dip to the eastward, which 

 brings up lower and. lower strata toward the foot-hills, indicates that 

 those at Fossil Ridge belong to a lower horizon than those which I 

 examined in the valley of the Cache a la Poudre. 



LIST OF CRETACEOUS POSSILS COLLECTED AT EOSSIL RIDCE, THREE 

 MILES SOUTHEASTWARD FROM SPRING CANON, AND AJBOUT SIX 



MILES SOUTH OF FORT COLLINS, COLORADO. 



1. Ostrea ■patina Meek & Hayden ? 



2. Pinna lalcesi White. 



3. Pier la I ingulf or mis Evans & Shumard. 



* Some remarkable Dinosamian remains have been obtained from the upper strata 

 of this group at the town of Morrison, on Bear Creek, Colorado. See address of Prof. 

 O. C. Marsh, Proe. Am. Assoc. Adv. Sci. vol. xxvi, pp. 22 et seq. See also further 

 remarks on a following page. 



