222 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The horizontally bedded breccias and lava flows extend eastward across 

 the head of Cache Creek to Index Peak and the range of peaks immedi- 

 ately south, but the lower portions of these mountains are irregularly 

 bedded. The upper parts, above 10,000 feet, consist chiefly of basalt flows 

 resting one upon another, with occasional intercalated layers of breccia. 

 This is also the case at the summit of the mountains northeast of Amphi- 

 theatre Creek, and southward to the summit of Mount Norris. 



The ridge between Cache Creek and Crandall Basin, with its western 

 spurs, is composed of nearly horizontally bedded breccias with few lava 

 flows. They carry numerous silicified trunks of trees, which are exposed 

 in a standing position on the western slopes of the Needles. Similarly 

 bedded breccias extend south of Cache Creek across Calfee and Miller 

 creeks, and form the plateau west of Lamar River and the high mountains 

 south of its head. Here, again, in the upper thousand feet massive basalt 

 flows prevail, forming the top of the plateau and the upper portion of the 

 peaks just mentioned. In this vicinity the basalt sheets are plainly seen 

 to slope gradually to the west and southwest, their highest present altitude 

 of about 11,000 feet being found on the summits of Castor and Pollux 

 peaks. Basalt sheets to the thickness of 400 feet cap the summit of Saddle 

 Mountain, at 10,400 feet altitude, where irregularly shaped flows of vesicular 

 and scoriaceous basalt indicate by their position and by the arrangement 

 of the columnar cracking that they flowed down an uneven surface, appar- 

 ently a drainage channel, sloping toward the southwest. 



Occasional flows of massive basalt occur in the lower portions of the 

 series of basic breccias and tuffs. A notable one is found near its base 

 over the limestone on Soda Butte Creek and Lamar River, but the great 

 bulk of the lava sheets is at the top of the series. Columnar structure is 

 common to all of these flows except when very thin and scoriaceous, and 

 they possess all of the superficial characteristics and variations of surface 

 flows of basalt. None of those observed in the localities just described 

 appear to have been intruded sheets. 



CHAOTIC BRECCIA. 



In striking contrast to the almost horizontally bedded breccias and 

 flows are the chaotic and absolutely orderless accumulations of scoriaceous 

 breccia which form the mountains and ridges about the head of Lamar 



