What metaphors are used in The Petrified Forest by Robert E. Sherwood?

In Act I, Gabby and Alan are having an indepth conversation about the power of nature, when Gabby realizes she is in over her head and changes the subject to Alan's destination. When Alan learns that the road outside leads to the Petrified Forest, he is delighted in the metaphor that an old fossil like himself should end up with other decaying life forms. Gabby doesn't like this line of conversation either and tells Alan that she would like to go to France with him.