white.] BOULDER PASS AND MIDDLE PAEK. 201 



posed of fragments of granitic and nietaniorphic rocks like those com- 

 posing tlie immediately adjacent mountains. These beds have been 

 much removed by later erosion, the approximately level portion, not 

 eroded, being from 200 to 300 feet above the neighboring streams. 

 Sometimes they break off by terrace slopes that are apparently not 

 caused by erosion. The higher surfaces of the deposit have a slight uni- 

 form slope toward the plains. It is difficult to estimate the thickness of 

 the deposit even approximately, and it is also difficult to ascertain 

 whether the stratified rocks upon which it was deposited were first lev- 

 eled off to receive the deposit, or whether the leveling was only of its 

 own upper surface. Its appearance suggests that it may have been de- 

 posited by a formerly existing ice-sheet moving off from the immediately 

 adjacent mountains, but there are some facts connected with it that are 

 difficult to explain in connection with that suggestion. High hogbacks 

 of Mesozoic rocks stand between those nearly level reaches of drift and 

 the granite rocks that furnished the material of which it is composed. 

 If the surface of the drift was really leveled off by an outwardly moving 

 ice-sheet, it is difficult to understand why the hogbacks were not also 

 reduced to the same plane. But they stand there, immediately adjacent, 

 several hundred feet above the surface of the drift, and also above many 

 of the adjacent granite foot-hills, and, so far as I could discover, they 

 show no signs of former glacial action upon them. 



Again, the source of the material of which the drift is composed is 

 only from two to ten niiles away, and yet its gravel and bowlders are as 

 perfect and smoothly rounded as the water- worn pebbles of a sea-shore. 

 They evidently have a history beyond that of mere detachment from 

 their original ledges and a few miles of glacial transportation. But this 

 subject will be again referred to on subsequent pages, though perhaps 

 not elucidated. 



Passing through the foot-hills near Boulder City, consisting mainly of 

 the* great hogbacks of the Bed Beds and Dakota Group, we left all the 

 sedimeutary rocks of the east side and traveled upon the great granite 

 nucleus of the Bocky Mountains until we had crossed the Front, or prin- 

 cipal rauge. Crossing this by way of Boulder Pass, we reached the 

 large, elevated intra-mountain region known as Middle Park. Our 

 journey led us into the park by way of the headwaters of Frazier Biver, 

 where we came upon the first stratified rocks after leaving the east side, 

 which were the " Bake Beds" of Dr. Hayden's reports. 



The geological structure of the park having been so ably reported 

 upon by the late Mr. Marvine, my attention was more especially directed 

 to the characteristics of the Baramie strata and the Bake Beds, with the 

 hope of learning something of their paleontological history. The latter 

 deposit is very extensively developed in the park and occupies a large 

 part of its surface. It rests unconformably upon all the other rocks, 

 from the granite to the Baramie strata inclusive. 



The strata (for it is distinctly stratified) generally presents a nearly 

 level aspect, but the original upper surface of the deposit has been 

 everywhere removed by erosion j so that of an original thickness of a 

 thousand feet or more, scarcely more than one-third of that thickness is 

 bow found at any one point. While the strata of this deposit have no- 

 where been so much displaced as all the other stratified rocks of the 

 park have been, they have, however, been in many places tilted at angles 

 varying from one to fifteen degrees. This deposit was carefully searched 

 for fossils at all the points which I visited, but without success except at 

 one point on Banch Creek, a tributary of Frazier Biver. Here I found 

 two imperfect specimens of a species apparently belonging to the genus 



