300 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



present valley for a short distance, and is therefore later than the period 

 in which the valley was eroded. The basalt is lig'ht gray and somewhat 

 vesicular, with small phenocrysts of olivine (1734, 1735). A similar basalt 

 occurs on Chipmunk Creek (1736). 



A small hill on the divide between Fox Creek and Mink Creek is 

 formed of a massive flow of pyroxene-andesite, which is jointed into great 

 slabs and is thinly fissile in places. It resembles many occurrences of 

 rhyolite. It is partly massive and vesicular, and is light gi'&y, with small 

 phenocrysts of feldspar and pyroxene (1727). It carries segregations of 

 feldspar and pyroxene, and has cavities containing tridymite (1726). Other 

 portions of the same lava sheet are darker and denser (1728, 1729, 1730). 

 The breccia along the western margin of the plateau is basic, and in places 

 there are remnants of porph} T ritic basalt. At the southern end of the valley 

 of Fox Creek it is composed of fragments of basic andesite in a light-red 

 matrix, and on the limestone hills east of the mouth of Crooked Creek it is 

 also basic. 



Isolated areas of basic andesitic and basaltic breccia occur overlying 

 very irregular surfaces of sedimentary rocks on the peak south of Pinyon 

 Peak, in the valley of Coulter Creek, in the vicinity of the northern end 

 of the Teton Range, on Berry, Boone, and Conant creeks, and at Birch 

 Hills. The last of these occurrences are described in connection with the 

 general geology of the northern end of the Teton Range (Chapter IV). 

 Those in the region of Coulter and Wolverine creeks and Pinyon Peak 

 are described by Mr. Arnold Hague in Chapter V, devoted to the descrip- 

 tive geology of Big Game Ridge and Huckleberry Mountain. 



Of 116 specimens from the upper basic breccia, one-half contain olivine 

 in variable amounts, and may be classed as basalt and basaltic andesite ; 

 the other half are free from it, and are pyroxene-andesites and hornblende- 

 pyroxene-andesites, the last-named rocks being much fewer than the pyrox- 

 ene-andesites. 



Hombiende-pyroxene-andesite. — The few specimens of this kind of andesite 

 which were studied prove to be glassy microlitic modifications, some with 

 dark-brown glass, one with red glass. Reddish-brown hornblende with 

 magnetite border, or an outer zone of pyroxene, magnetite, and plagioclase, 

 is the usual variety. In one rock brown hornblendes without any border 

 of foreign material occur so closely associated with pyroxene as to inclose 



