56 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



of the rocks in situ in that region, it is evident that the original ledges 

 from which they have been separated are those of the axes of the 

 neighboring mountains. Therefore it is equally evident that they have 

 been distributed in radiating lines from those axes ; especially so since 

 in every case yet observed, the differences in the lithological composition 

 of the drift-pebbles at different localities adjacent to the mountain range 

 exactly correspond to the changes of lithological characters of the rocks 

 comprising different portions of the mountain axis. That these drift- 

 pebbles were distributed by glacial action seems, in view of the accepted 

 theories in relation to the great northern drift, to be a natural inference 

 upon casual examination ; especially as this mountain drift is distributed 

 upon the surfaces of the higher foot-hills of the mountain ranges, as well 

 as in the valleys between them, and upon the lower lands at a greater 

 distance. The true origin and distribution of this drift, however, 

 involves the consideration of several questions, for the discussion of 

 which the phenomena observable within the limits of this district do not 

 furnish sufficient material. Therefore the consideration of the subject 

 in this report will be mainly confined to the description, and an account 

 of the distribution of the drift within this district. Before doing this, 

 however, it is proper to refer briefly to some correlated phenomena. 

 The drift-pebbles, as before mentioned, are as smoothly rounded as any 

 that are now washed upon the sea-coast ; and it seems difficult to 

 understand how this could have been done under any condition of trans- 

 portation alone. Indeed, it seems impossible that these pebbles could 

 have been so completely rounded in any manner except by attrition in a 

 large body of water where the waves could have great power; and this 

 view is strengthened by the fact that the pebbles are distributed over so 

 broad an area. Whatever their origin as such may have been, it is cer- 

 tain that the pebbles have been radiately dispersed from the mountain 

 axes. 



The character of the rocks composing the axial portions of the Uinta 

 and Park Eange uplifts respectively, is so different that it is quite easy 

 to refer any collection of pebbles that may be found in the vicinity of 

 both ranges to the one from which they were really derived, and also to 

 recognize any mixture of pebbles as such that have been derived from 

 both ranges. For example, it is easy to recognize the drift-pebbles of 

 the eastern portion of this district as having been derived from rocks of 

 those mountains of the Park Range which lie to the eastward ; and just 

 as easy to recognize those that are found in the western part of the dis- 

 trict as having been derived from the strata of Uinta Mountain uplifts. 



Upon the middle portion of the district, however, the drift of both 

 eastern and western origin ; or, in other words, thatof the Park Eange, 

 and of the eastern end of the Uinta Mountains, is found to be mixed. 

 The former is mostly quartzites, with admixtures of basalt and granite, 

 while the latter are mostly made up of fragments of carboniferous lime- 

 stone and Weber quartzite, which latter rock is often, but not always, 

 quartzitic. This Uinta drift contains no trap and no granite, for none 

 of those rocks occur in the eastern portion of the Unita Mountains. 



Granite, however, is abundant in the Park Eange, and very extensive 

 masses of the basalt exist just beyond the eastern borders of the dis- 

 trict. The pebbles of eastern drift which have found their way in 

 abundance into the beds of the White and Yampa rivers, have been 

 carried down by the current to points further westward in the valleys 

 than any they have reached upon the uplands, and further than any 

 points to which drift from the Uinta Mountains has been transported 

 eastward. 



