no ELIOT BLACKW ELDER 



probably washed down from its points of origin by temporary 

 hill streams, sifted from the coarser fragments, imperfectly sorted 

 mineralogically, and soon deposited on the fiat bottoms of broad, 

 intermontane basins. It is a suggestive fact that the sandy silt 

 taken from a cut bank in the broad fioodplain of Owl Creek has the 

 same structural, textural, and mineral characteristics as some of the 

 Wind River strata. This silt is a modern fioodplain deposit in a 

 drainage basin which does not contain outcrops of the typical Wind 

 River strata. A mere reworking of the Eocene material in this 

 instance is scarcely possible. 



Some parts of the Tertiary sequence are obviously volcanic in 

 origin. Thus, along the east side of Jackson Hole, there are 

 considerable flows of andesite and rhyolite in the lower part of the 

 formation. If we may rely upon lithologic similarity and field 

 relations, we may correlate with these the flows which cloak the 

 west slope of the Teton Range, although the relationship is not yet 

 firmly established. At the northwest end of the Wind River Range, 

 where it articulates with the mountains of Yellowstone Park, thick 

 beds of volcanic ash and agglomerate with interbedded glassy 

 lava flows rest upon the pre-Tertiary folded rocks, but are them- 

 selves younger than the Wind River Eocene. Traced eastward 

 to Horse Creek, the Washakee Needles, and the valley of Owl 

 Creek, this thick volcanic series is found to rest conformably upon 

 the striped clays of the Wind River formation, with which they inter- 

 grade through gray, plant-bearing shales and greenish volcanic 

 sandstones containing petrified logs. A closer examination of the 

 volcanic beds shows that some of them are massive agglomerates, 

 devoid of stratification, whereas other beds are distinctly stratified, 

 cross-bedded, and occasionally interrupted by lenticular sheets of 

 coarse gravel, suggestive of stream channels. The conditions indi- 

 cated are those which would be found upon low gradient river plains 

 adjacent to active volcanoes. 



In order to place these conditions as closely as possible in 

 chronology, it would be necessary to know the ages of all parts of 

 this Tertiary sequence. Unfortunately, this is not fully under- 

 stood. On the basis of freshwater shells, and land plant leaves 

 which were found in the valley of the Gros Ventre River by the 



