434: GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



and rhyolite found on the plateau ridge 1 mile west of Beaver Lake (1804 

 et seq.) and just described in connection with the rhyolite. 



The basalt sheets along Gardiner River, around the base of Bunsen 

 Peak, and at Osprey Falls, and those in the valley of Lava Creek and at 

 Undine Falls and in this vicinity, occur partly overlying rhyolite, partly 

 beneath it. At Osprey Falls there are eight horizontal sheets of columnar 

 basalt directly overlain by a similar sheet of columnar rhyolite, which m 

 turn is overlain by a horizontal sheet of columnar basalt. Owing to the 

 dark color of the surface of the columns of rhyolite, all of the lava sheets 

 appear alike when seen from a short distance. The rhyolite sheet is a thin 

 edge of the great sheet of the plateau south. The petrographical character 

 of the basalts at this place is like that of those just described — dark gray, 

 with abundant minute feldspars and occasionally larger ones of microtine. 

 The same is true of the basalts in the valley of Lava Creek and of those 

 • at Undine Falls. Here the position of the sheets is like that of those at 

 Osprey Falls, but their relation to the rhyolite is not so plain. Just north 

 of this valley, along the crest of the south face of Mount Everts, there are 

 several exposures of basalt directly underneath the rhyolite sheet which 

 tops the mountain. The basalt is dark colored, vesicular, and scoriaceous, 

 with few small phenocrysts of feldspar and pyroxene (618 to 621). In 

 another exposure it is more crystalline and vesicular, with numerous 

 phenocrysts of medium-sized microtine, and a few of olivine (622). 



Remnants of basalt sheets occur on both sides of Yellowstone Valley 

 from near Gardiner to Broad Creek, and also up the Lamar Valley. In 

 some cases the time of their eruption, relative to that of the rhyolite, can 

 not be learned. But in most cases their position with respect to other 

 formations is such as to correlate them with the recent basalts. Petro- 

 graphically also they resemble them. 



In the neighborhood of Tower Falls basalt forms a massive sheet rest- 

 ing directly upon andesitic breccia, and has the kind of columnar structure 

 characteristic of surficial lava streams. On the west side of the canyon, 

 north of the falls, it forms a vertical cliff 150 feet high, with beautifully 

 developed columnar structure, shown in PL LVIII. The lowest part con- 

 sists of dense basalt, cracked into vertical columns, 2 to 2 J feet wide and 5 

 to 12 or 15 feet high. These merge upward through continuous rock into 

 small, slender columns, which curve slightly toward centers near the top of 



