of the Yellowstone Parlt. 453 



On both sides of the Yellowstone, stretching from Crescent 

 Hill to Junction Butte, occur several isolated exposures of an 

 acid lava, remnants of a much more extensive body, the 

 greater part of which has been carried away by erosion. Along 

 the Yellowstone "Valley denudation of the overlying material 

 has been so excessive that the relations of these acid rocks to 

 other igneous rocks are somewhat obscure. In many instances 

 it would be impossible to make out their geological position, 

 as they lie completely isolated from other lavas. In some 

 instances they have been preserved from erosion by cappings 

 of basalt, but the age of these basalt flows is by no means iden- 

 tical. They have been grouped together and designated trachy- 

 tic-rhyolite, from certain resemblances in mineral composition 

 to both these types of igneous rocks, although it can not be 

 stated definitely that they all belong to the same period. For 

 the present they have been correlated as being contemporaneous 

 in age, and are regarded by the writer as of later age than the 

 series of early acid and basic breccias. 



As regards their geological relations to other eruptive masses 

 and the general sequence of lavas, they play an unimportant 

 part, and for the purposes of the present paper, which deals 

 more especially with the age and duration of volcanic energy 

 in this region, they are of slight interest, being limited in 

 amount, covering small areas, and carrying no evidences of a 

 fossil flora. 



Resting directly upon the irregular surfaces of the early basic 

 breccias comes a succession of distinctly bedded basalt flows, 

 which in places have attained an aggregate thickness of nearly 

 1,000 feet. They cap several of the higher ridges of the 

 Absaroka Range, and on the west side of the Lamar Valley 

 west of the Range, they present a grand escarpment forming 

 the abrupt wall of Mirror plateau. From this plateau the 

 basalts stretch southward and eastward, sharply defining the 

 series of early acid and basic breccias from later accumula- 

 tions of igneous rocks. These basalts have been designated 

 early basalt flows, as they mark a distinct period in the geolog- 

 ical history of the volcanic eruptions, although flows of basalt 

 frequently occur in the older breccias. 



Following these early basalts comes a second series of acid 

 and basic breccias similar in mode of occurrence, mineral com- 

 position, and in the varying manifestations of the different 

 phases of eruptive energy. Apparently the centers of volcanic 

 action moved southward and the later series of acid and basic 

 breccias built up the western portion of the southern half of 

 the Absaroka Range. Unlike the early acid breccias, the late 

 acid breccias are not so deeply buried beneath the more recent 

 lavas, but form the tops of several high peaks, and cover large 



Am. Jour. Sci. — Fourth Series, Yol. I, No. 6. — June, 1896. 

 30 



