454 A. Hague — Age of the Igneous Rocks 



areas along the summit of the range. The late basic breccias 

 buried in great part the late acid breccias, and in turn built up 

 the extreme southern end of the Absaroka Range, or at least 

 that portion lying within the limits of the Yellowstone Park. 

 The basic breccias stretch southward far beyond the bounda- 

 ries of the park, and constitute the top of the Wind River 

 plateau, which serves to unite the Wind River and Absaroka 

 Ranges. In places the late basic breccias are shown in precip- 

 itous cliffs, exposing a thickness of over 2,000 feet of volcanic 

 material. Unlike the early acid and basic breccias, neither the 

 late acid nor basic breccias have as yet yielded any valuable 

 contribution to our knowledge of an extinct flora. Fragments 

 of silicified wood are common enough, but as yet no bed of 

 tuffs or fragmental material has been found to carry a well- 

 preserved flora. Occasionally fragmental impressions of stems, 

 twigs, and leaves have been collected, but they are too poor for 

 specific identification and of no special significance for com- 

 parative purposes. It may be said that the Absaroka Range 

 has nowhere as yet afforded any important plant-bearing beds, 

 the localities furnishing the material lying to the westward of 

 the range. 



Penetrating both the late acid and basic breccias, and conse- 

 quently of later age, occurred extensive outflows of hornblende- 

 mica-andesite. As these dense rocks withstand atmospheric 

 agencies better than the underlying breccias, they often form 

 the summit of isolated peaks in the more elevated portions of 

 the range. These hornblende-mica-andesite flows denote a 

 distinct period in the phases of volcanic phenomena in the 

 Absaroka Range*. 



After the cessation of volcanic energy which built up the 

 vast pile of andesitic and basaltic breccias and flows, a long 

 period of erosion followed. An enormous amount of material 

 was swept away. Broad valleys and deep canyons were carved 

 out and the present mountains outlined somewhat as they 

 appear to-day. Then with renewed activity volcanic eruption 

 again broke out and immense masses of rhyolite were 

 ejected. These rhyolite flows changed the depressed 

 basin lying between the mountains which surround the 

 Park into an elevated plateau, the lavas accumulating to a 

 thickness of nearly 2,000 feet. The appellation of Park Pla- 

 teau has been given to this striking physical feature which em- 

 braces an area not less than 50 miles in length and 40 miles in 

 width. On all sides the lower slopes of the pre-existing ranges 

 were buried beneath rhyolite, and in some instances the deeply 

 eroded valleys in the andesitic breccias of the Absaroka range 

 were partially tilled by the later rock. Underlying the rhyo- 

 lite at one or two localities occur limited flows of basalt and 



