THE MIOCENE PERIOD. 



271 



at least intermittently, through the Eocene, made itself felt also in 

 the Miocene, and perhaps reached its maximum toward the end of 

 that period. The frequent references in preceding pages to igneous 

 materials in the sedimentary formations of the system give some 

 idea of the extent of Miocene vulcanism. The eruptions were from 

 fissures as well as from volcanoes, and extensive sheets of lava as 



Fig. 449. — Monument of Gering (Miocene) sandstone over Brule (Eocene) clay, western 

 Nebraska. (Darton, U. S. Geol. Surv.) 



well as volcanic cones were formed, and intrusions as well as extru- 

 sions were of frequent occurrence. Evidences of volcanic activity 

 during this period are found in nearly or quite every State west of 

 the Rocky mountains. Among other centers of igneous activity may 

 be mentioned the basin of the Columbia l and the Yellowstone National 



1 Landes, Wash. Geol. Surv., Vol. II, and Smith, G. O., EUensburg folio, U. S 

 Geol. Surv. 



