A classic sort of Hawthorne tale:
- It started from a line or two in his notebook. (His notebooks are full of ideas.)
- It features a fiercely and sternly religious person.
- There's an elaborate metaphor.
Or maybe not so elaborate: Richard Digby is a hard-hearted man who believes that he's got the true faith, so he leaves society, and finds a cave where the water seems to petrify everything. Oh, also, he's got a medical condition where his heart is hardening.
So maybe we shouldn't call the metaphor here "elaborate"; it's more like "belabored." How many ways can you connect a guy to "hard" and "stone"? He's petrifying on the outside, while calcifying on the inside. Meanwhile, the petrifying quality of the cave isn't exactly hidden.
From that, you might consider this story a little obvious--and it is, but that's not necessarily a bad point. Back when Hawthorne was writing, this sort of sentimental and obvious tale might be pleasing to people; today, you can read it with a slight wink; and honestly, it's not clear that there wasn't a wink here this whole time.