BASALT FLOWS AND INTRUSIVE IGNEOUS ROCKS
  Yellowstone area--Includes Osprey, Madison River, Swan Lake Flat, and Falls River Basalts,
     basalts of Mariposa Lake, Undine Falls Basalt, and gravels, sands, silts, and basalts of
     The Narrows. Qb
  In and adjacent to Absaroka and Washakie Ranges--Includes basalt of Lava Mountain (age about
     0.5 Ma). Qb
RHYOLITE FLOWS, TUFF, AND INTRUSIVE IGNEOUS ROCKS--Includes Plateau Rhyolite
     (age about 0.07 Ma) and interlayered sediments, Mount Jackson Rhyolite (age 0.6 to
     about 1 Ma), Lewis Canyon Rhyolite (age about 0.9 Ma); and Lava Creek Tuff of
     Yellowstone Group (age 0.6 to about 1 Ma). Qr
ALKALIC EXTRUSIVE AND INTRUSIVE IGNEOUS ROCKS (AGE 1.25 Ma)--Leucite- and
     Nepheline-rich flows, scoria, and necks. Qi