Formal
Again, think university lecture here. Campbell wants to deliver the information as expediently as possible, and dude doesn't have time to worry about getting all flowery or poetic.
He dips heavily into the three-dollar words in order to be specific (and probably to show people he's extra-smart), and he wanders into a lot of cul-de-sacs with his examples, but he isn't trying to convey a specific emotion or states of mind. He wants to illuminate the purpose of all those myths and fairy tales.
Formal expression is the name of the game. The more personal style belongs to the stories he's analyzing:
And we know that the choral songs (dithyrambs) and dark, blood-reeking rites in celebration of the god—associated with the renewal of vegetation, the renewal of the moon, the renewal of the sun, the renewal of the soul, and solemnized at the season of the resurrection of the year god—represent the ritual beginnings of the Attic tragedy. (131.1)
Pretty heavy, isn't it? Ironically, none of the examples helps much in that department: they make the point…but easy reading this ain't.