"I knew the Pythia had chewed laurel leaves"
Apollo with a laurel wreath
See bookmark to Page 39. " as the Oracle at Delphi said to the Spartans ".
The Pythia were the prophetic priestesses at Delphi. Although their kind of divine madness was described as prophetic rather than Dionsysian, there is a link to Dionysus as Apollo only occupied the temple at Delphi for nine months of the year; during the other three, Dionysus came to the site. However, it isn't known whether the Pythia would engage in Dionysian rituals while the latter was present.
The laurel tree was considered sacred to Apollo. Visitors to Delphi who sought advice from the Oracle would approach the temple carrying laurel branches, and Apollo is often depicted with a laurel wreath on his head. Richard notices the boiling pot of laurel leaves on p. 102, when he is staying with the others at Francis's country house.