.0                                                                              
                  IGNEOUS ROCKS OF TERTIARY AGE                                 
                                                                                
Tbb    BASALT FLOWS AND ASSOCIATED TUFF, BRECCIA, AND CONGLOMERATE OF LATE-     
         VOLCANIC BIMODAL SUITE (AGE 3.5-26 M.Y.)--Includes basalts of          
         Hinsdale Formation in San Juan Mountains, Servilleta Formation         
         in San Luis Valley, and many other occurrences                         
Tbbi   BASALTIC INTRUSIVE ROCKS RELATED TO BASALT FLOWS (Tbb)--In dikes and     
         plugs                                                                  
Tbr    RHYOLITIC INTRUSIVE ROCKS AND FLOWS OF LATE-VOLCANIC BIMODAL SUITE       
Tbrt   ASH-FLOW TUFF OF LATE-VOLCANIC BIMODAL SUITE (AGE 22-23 M.Y.)            
Taf    ASH-FLOW TUFF OF MAIN VOLCANIC SEQUENCE (AGE IN SAN JUAN MOUNTAINS       
         26-30 M.Y.; IN SOUTH PARK 29-32 M.Y.)--Includes many named units       
Tial   INTRA-ASH FLOW ANDESITIC LAVAS