188 GEOLOGY OF IDE BLACK UlLLS. 



of the Fort Union beds and tlie history of the Hills. Tlie Wliite River 

 beds, however, liave been definitely connected with that history, and of 

 them we are fortunately able to say that their Miocene age is not called in 

 question. Dr. Leidy and Professor Marsh, who liave independently ex- 

 amined their profuse vertebrate fauna, are at one in their determination of 

 age, and Professor Meek finds nothing in the scanty fresh-water molluscau 

 fauna to conflict with tlieir conclusion. 



The partial section of the Fort Union group observed by the writer on 

 Old Woman Fork has been described in the preceding chapter and needs 

 no further mention. 'i1iat chapter contains also a description of so much 

 of the White River group as was seen by our party. In the latter part of 

 the last section a description will be found of the lower strata of the group 

 as they a[)pear resting on the Cretaceous near the mouth of Rapid Creek. 



The bed of gravel at the base of that exposure of the White River 

 beds is only one of many evidences that there was an interval of time 

 between the deposition of the upper Cretaceous beds and the deposition of 

 the White River. The fact tliat they are in juxtaposition at this point, 

 while to the west of the Hills the Fort Union group is interposed between 

 them, is another evidence, and a third is derived from the disparity of age 

 of the two formations as shown by Jieir fossils. It was during this interval 

 of time that the principal uplift of the Hills occurred, and one of the chief 

 witnesses to that fact is the same conglomerate at the base of the upper 

 series. 



The pebbles of the conglomerate are never more than two or three 

 inches in diameter, and have such a large preponderance of white quartz 

 that the whole bed has a very light color. It is so loosely cemented that 

 outcrops are merely piles of loose pebbles, having all the appearance of a 

 gravel beach on the sea shore. Indeed, it is evidently a shore deposit, the 

 remains of the beach of the old fresh- water lake, formed before the waters 

 attained their greatest height and while they were sorting over material 

 brought by rivers from tlie neighboring Black Hills. Nearly the whole of 

 the material seems to have been derived from the quartzes and quartzites 

 of the various formations now exposed there, but Mr. Jenney succeeded in 

 finding, mingled witli these, not only the rose-quartz which is the com- 



