J. Grotzinger

5) Vug-Sphere Relation.  In the lower left, a spheroid can be seen
   penetrating the interior of a vug.  Geologists refer to this as a
   "cross-cutting" relationship, which allows the relative timing of
   separate events to be established.  In this case, the spheroid
   appears to "invade" the vug, and therefore likely post-dates the
   vug.  Very likely it grew after the mineral which filled the vug
   had precipitated, but before that mineral had been removed to form
   the vug.  This suggests that the spheroids, if concretions, may
   have been one of the last features to form within the outcrop.