So Dorcas knows nothing about that black beard, said Poirot thoughtfully, as we walked out into the hall again. 
Poirot said that Dorcas was very knowledgeable about the black beard. 
mnli hypothesis: Poirot said that Dorcas was very knowledgeable about the black beard.  premise: So Dorcas knows nothing about that black beard, said Poirot thoughtfully, as we walked out into the hall again. 