stratigraphic intervals
   (System) †Cambrian (541.0 - 485.4 Ma)
     (Series) †Middle (509 - 497 Ma) see Series Series 3 (509 - 497 Ma)
       (Stage) †mid Middle
         1 (Substage) †upper
         2 (Substage) †[middle]
         3 (Substage) †lower