202 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



Helix; and imbedded in the same mass a small metacarpal or metatar- 

 sal bone, about one centimeter long, and apparently belonging to a small 

 rodent. These give no indication of the age of the deposit, because 

 they belong to common living types, and because fossil forms of living 

 invertebrate types are abundant in strata as old as those of the Laramie 

 Group. We must, therefore, rely upon other phenomena to indicate the 

 age of this deposit. The following is a summary of the conditions and 

 indicated circumstances bearing upon this point: 



The Lake Beds are known to be considerably more recent than the 

 Laramie Group, because the former rests conformably upon the flexed 

 and much-eroded strata of the latter. The deposit is a circumscribed 

 one, evidently of fresh-water origin, occupies only the lower surfaces of 

 the park, and has plainly derived its material from the immediately 

 adjacent and siurounding hills, but doubtless before either the deposit 

 or the hills were elevated to their present height above the level of the 

 sea. On the other hand it is known to be quite ancient as compared 

 with the present time, because it has suffered extensive erosion, evi- 

 dently amounting to more than half its original bulk. It bears the 

 evidence of drift-leveling similar to that which has been already noticed 

 as occurring at the eastern base of the mountains, and is consequently 

 older than the epoch of that drift ; and its strata have been considerably 

 flexed in some places, showing that mountain elevation was continued 

 after its deposition. This deposit is regarded as of the same or about 

 the same age as the one found west of the mountains which has been 

 called the Brown's Park Group by Professor Powell, and Uinta Group 

 by Mr. King. A comparison of the two was made in my report of last 

 year, and reference to the subject will necessarily be made on following 

 pages. 



The drift phenomena observed in Middle Park consist of scattered 

 gravel and bowlders upon quite extensive, nearly level, or gently slop- 

 ing surfaces of different heights, which often assume the character* of 

 terraces. The gravel and bowlders are sometimes scantily and some- 

 times profusely spread, and the material of which they are composed 

 has evidently been derived from the immediately adjacent mountains and 

 hills. The pebbles and small bowlders have been as smoothly rounded, 

 evidently water worn, as those which have already been referred to as 

 existing at the eastern base of the mountains. The leveling of the sur- 

 faces upon which this material rests may have been produced by the same 

 forces which scattered it, but the evidence on this point is not conclu- 

 sive. The various aspects in which one may view these surfaces in 

 different portions of. the park suggest the idea that they may repre- 

 sent different base-planes of erosion which were successively reached 

 during the process of erosive excavation of the deposits which formerly 

 more completely than now filled the park. This terracing of the depos- 

 its of the park is known to be of comparatively recent date, because they 

 occur upon the older and later deposits alike, and even the higher or 

 older terraces or levels are, in many cases, upon the Lake Beds, which 

 are much the latest of the formations. 



In Egeria Park, which lies west of Middle Park and of the Park Eange 

 of mountains, these drift terraces or levels are very extensive, and con- 

 stitute even more conspicuous features of that district than they do in 

 Middle Park, but they are there mainly or wholly carved out of the Cre- 

 taceous deposits, there being no later deposits within its limits. 



The strata of the Laramie Group are abundantly developed in Middle 

 Park, so far as aggregate thickness is concerned, which is fully double 

 that of the strata of the same period in the region which I examined 



