The narrator describes the appearance of the snow and the distant trees of the Yukon, saying that

it was an unbroken white, save for a dark hairline that curved and twisted from around the spruce-covered island to the south.

By referring to the line of trees as a hairline, he uses a metaphor to compare them to this feature of a human head. A metaphor is a comparison of two unlike things where one is simply said to be the other.

The narrator describes the tobacco spit that freezes to the main character's facial hair as being "like glass" because, if it fell from his face, it would shatter into brittle fragments. Here, the narrator uses a simile, a comparison of two unlike things where one is said to be like the other using the words like or as. This simile seems to emphasize the man's relative weakness: how fragile he is compared to the natural world around him, which seems vast and so powerful.

The narrator uses another metaphor to describe the man's thinking about the springs under the snow, the springs that never freeze. "They were traps," he thinks, because they hide pools of water under the snow, and if one steps into such a pool, wet feet would necessitate stopping to build a fire to dry them out before one's feet freeze. Another metaphor describes the dog's instincts when it falls into one of these pools. The dog tries to bite the ice from its toes, following the "mysterious prompting that arose from the deep crypts of its being." A crypt is usually an underground vault where the bodies of the dead are placed.

There are many similes and metaphors of this nature throughout the story.

While Jack London's "To Build a Fire" is not filled with similes and metaphors (more personifications exist), one can find a few instances.

Metaphors:

It meant life, and it must not cease.

Here, the metaphor exists in the reference to "it." "It" is being referred to as the fire, although one could see that the man is also comparing the fire to life. Although one is directly dependent upon the other, the man does not want either to "not cease": neither life nor the fire.

The fire provider had failed.

Here, the metaphor exists in the comparison of the fire provider to a few possible things. First, it could be the man. He provides material to build up the fire, Second, it could be the twigs he places into the fire. They are needed to catch fire to keep it burning. Lastly, although this is a bit of a stretch, it could be Nature itself. Given that London was a Naturalistic writer, he could be referencing Nature (personified) as the fire provider.

Similes:

It was like hearing his own judgment of death.

Here, the snow has just fallen on the man's fire, extinguishing it. The "it was like" aspect compares the sound of the snow extinguishing the fire to the sound of death coming for him.

SIMILES

• "The ice held his lips so tightly together that he could not empty the juice from his mouth. The result was a long piece of yellow ice hanging from his lips. If he fell down it would break, like glass, into many pieces."--This is a description of the ice that is forming on the man's face from the tobacco juice.  The simile here emphasizes the fragility of the ice; it would shatter just like glass shatters.
• "The man was shocked. It was like hearing his own judgment of death. For a moment he sat and stared at the spot where the fire had been."--This simile is foreshadowing the man's death as the fire goes out. It's one of the first times that the man understands the danger of his journey.

The lack of many similes and metaphors is a reflection of the "lack of imagination" that Jack London uses to describe the man's outlook on life. Had he been able to imagine the extreme conditions in which he set out on his journey, he would not have died in the end. While the dog is an animal and therefore also lacks an imagination, it knows to follow animal instincts, like when it falls through some ice and gets its paws wet. The dog shakes off the ice and then chews the rest off because its instincts tell it to. The man, because he is human, ignores his animal instincts, which is another fatal flaw to his character.

SIMILES 

• "Once, coming around a bend, he shied abruptly, like a startled horse..." -- London compares the man's abrupt stop to that of a "startled horse."
• "...the thick German socks were like sheaths of iron half-way to the knees..." -- The socks are being compared to iron.
• "...the moccasin strings were like rods of steel all twisted and knotted as by some conflagration." -- The strings of the moccasin are being likened to steel.
• "...and he wondered if Mercury felt as he felt when skimming over the earth." -- The man is comparing his own lightness of being as that of Mercury soaring above the earth.
• "...running around like a chicken with its head cut off—such was the simile that occurred to him."

METAPHORS

• "The blood was alive, like the dog, and like the dog it wanted to hide away and cover itself up from the fearful cold." -- There are a pair of metaphors here: The blood is being compared to the living dog; the blood is also being shown its desire to hide itself from the cold as would the dog.