BEDDED BASIC BRECCIA. 221 



there is a system of narrow ridges and valleys, three of the deepest valleys 

 passing through what was the center of the volcano. The arrangement of 

 the rocks, however, is so marked that there can be no doubt about the loca- 

 tion of the center of volcanic activity or of the general nature of the 

 mountain. 



DISTINCTLY BEDDED BRECCIA. 



The geologist who approaches the region by way of the Lamar River 

 is impressed with the great masses of almost horizontally bedded breccia 

 which form the chocolate-brown mountains on either side of Soda Butte 

 Creek. They are finely shown in the panoramic sketch by Prof. W. H. 

 Holmes in his report on the geology of the Yellowstone Park, 1 notices of 

 this portion of the country being found on pages 44 to 49. The sketch is 

 as faithful to nature as it is artistic, and it is possible to calculate the point 

 from which it was made by reference to the map. 



The breccias rise from 2,000 to 3,000 feet above the river, and appear 

 so uniformly bedded as to give the impression that they have been water- 

 laid; but a nearer view shows their irregularity and proves their subaerial 

 deposition. Upon examination the bedding is found to be crude and ill 

 defined, the layers consisting of tuff with various-sized fragments of scoria 

 and compact rock. Between the layers are occasional sheets of massive 

 lava. In some places the tuffs are quite light colored and are very notice- 

 able among the dark-brown beds. The deposits contain the stumps and 

 roots of trees, whose erect position shows that they have not been disturbed 

 since they were buried beneath showers of dust and stones. Their situa- 

 tion at different horizons in the breccia and their larg-e size indicate how 

 great a time must have elapsed between the explosions which covered the 

 country with debris in beds from 1 to 3 or more feet in thickness. 



A more or less distinct and nearly horizontal bedding is characteristic 

 of the breccias forming the mountains west of Cache Creek, including the 

 ridge from Mount Norris through The Thunderer to the great wall sur- 

 rounding Amphitheatre Creek. The east face of this ridge is shown in 

 PI. XXIX, from a photograph which was taken from the divide at the head 

 of Republic Creek. The same breccias form the precipitous ridges on both 

 sides of Pebble Creek, and are well exposed in the face of Baronett Peak, 

 10,300 feet in altitude. 



'Twelfth Ann. Rept. U. S. Geol. and Geog. Surv. Terr., Part II, 1883, pp. 1-62. 



