Treak Cliff cavern does have Blue John but there is a separate cave called Blue John Cavern or Blue John Mines. Along with the Speedwell cavern (done by boat) and the afore mentioned Peak Cavern they make up the 4 show caves in Castleton. There are dozens of others that link to the same system and great inroads were made not too long ago by those hardiest of all potholers - The explorers and blasters - Linking Titan Cave and, I think, Speedwell.