In the article " Where I lived and what I lived for " explain Thoreau figurative language use of the word burrowing

The figurative langauge used by Thoreau in the sentence that includes the word "burrowing" is a simile.

My instinct tells me that my head is an organ for burrowing, as some creatures use their snout and fore-paws, and with it I would mine and burrow my way through these hills.