Twain: "transposition of epochs" to the far past. Wells: time travel to the far future. Heinlein: circular causality in the near and far futures. Harrison and Powers: circular causality in historical periods. Matheson and Niffeneger: circular causality in the characters' lifetimes. De Camp and Moore: causality violation in historical periods. Anderson: both causality paradoxes in many future and historical periods; circular causality in Jack Havig's lifetime.
Whichever way I look at time travel, I find Poul Anderson incorporating and transcending the achievements of other authors.