WHITE] BOULDER PASS AND MIDDLE PARK. 201 



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

 posing the immediately adjacent mountains. These beds have been 

 miicli 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. Tlie liigher surfaces of the deposit have a slight uni- 

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

 the deposit even appR)ximately, and it is also difficult to ascertain 

 whether the stratifie<l rocks upon Avhich it was deposited were first lev- 

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

 0"wni up])er surface. Its appearance suggests that it may have been de- 

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

 adjacent mouutains, but there are some facts connected with it tliat 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' bj'^ 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 1 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 miles away, antl yet its gravel and bowlders are as 

 l^erfect 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 vvill 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 lied Beds and Dakota Group, we left all the 

 sedimentary rocks of the east side and traveled ujjon the great granite 

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

 cipal range. Crossing this by way of Boidder Pass, we reached the 

 large, elevated iutra-mountain region known as Middle Park. Oiu' 

 journey led us into the park by way of the headwaters of Prazier Eiver, 

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

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



The geological structiu-o of the park having been so ably reported 

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

 to the characteristics of the Laramie strata and the Lake 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 

 ])art of its suiiace. It rests unconformably upon aU the other rocks, 

 from the granite to the Laramie 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 

 every Vvhere removed by erosion ; so that of an original thickness of a 

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

 now found at any one point. While the strata of this deposit liave no- 

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

 park have been, they have, however, been in many i)laces tilted at angles 

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

 for fossils at all the points which I \'isited, but without success except at 

 one point on Eancli Creek, a tributary of Prazier Eiver. Here I found 

 two imperfect specimens of a species api)arently belonging to the genus 



