220 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



mixed with some pyroxene-andesite. It is also exposed in the valley of 

 Cache Creek in two places, the larger exposure extending for about 8 miles. 

 Here it is light colored and consists of the same kinds of andesite. It is 

 indurated and somewhat decomposed, and its surface indicates that it was 

 eroded before the basaltic breccia was thrown on it, A much smaller body 

 of hornblende-mica-andesite-breccia occurs in the heart of the district within 

 a mile of the center of the volcano. It is at the junction of Closed Creek 

 and Timber Creek, and rests directly on limestone, as it also does in the 

 vicinity of Republic Creek. It appears to have been at one time on the 

 outskirts of the earlier volcanic district, for its lowest portion is composed 

 of layers of andesitic gravel which were deposited by water. It passes up 

 into light-colored breccia of hornblende-mica-andesite, which carries frag- 

 ments of Archean rocks. Fragments of gneiss and schist characterize the 

 early acid breccia wherever it has been found along the northern boundary 

 of the Yellowstone Park. They have already been mentioned as occurrinff 

 in that which forms the base of Sepulchre Mountain, and they occur in that 

 at the base of the Washburne volcano. This early acid breccia has been 

 shown by Mr. Hague 1 to belong to the Eocene period, and to correspond to 

 the Fort Union horizon. Throughout the remainder of the district the 

 basaltic breccia rests directly on the sedimentary strata, or forms the bottom 

 of the valleys where erosion has not yet cut through them to the underlying 

 rocks. It is to be remarked that the basaltic lavas pass under a second 

 series of acid breccias of hornblende-mica-andesite, which are like the older 

 ones in mineralogical character. The younger or late acid breccias form a 

 considerable part of the Absaroka Range south of Lamar River. Neither 

 the older nor the younger of these accumulations of hornblende-mica- 

 andesite appears to have been erupted from what we have called the volcano 

 of Crandall Basin. This was essentially a basaltic center, the last eruptions 

 of which became acid, and in part more basic, but were of small extent. 



BASIC BRECCIA AND FLOWS. 



A conception of the magnitude and proportions of this volcano must 

 be derived from a study of the geological structure of the basaltic breccia 

 and flows — early basic breccia — for nothing remains to indicate a single 

 line of the original form of the mountain. In place of a volcanic cone 



1 Hague, Arnold, The age of the igneous rocks of the Yellowstone National Park: Am. Jour. Sci., 

 4th series, Vol. I, 1896, p. 450. 



