Symbolism, Imagery, Allegory
You already know that The Hours has some things to say about the toll that time takes on our short human lives, and, throughout the novel, Michael Cunningham occasionally drops an image that draws attention to the seemingly endless passage of time. Check out this passage from the novel's first chapter, for instance:
She still has a certain sexiness; a certain bohemian, good-witch sort of charm; and yet this morning she makes a tragic sight, standing so straight in her big shirt and exotic shoes, resisting the pull of gravity, a female mammoth already up to its knees in the tar, taking a rest between efforts, standing bulky and proud, almost nonchalant, pretending to contemplate the tender grasses waiting on the far bank when it is beginning to know for certain that it will remain here, trapped and alone, after dark, when the jackals come out. (1.8)
Now, Clarissa Vaughan isn't a particularly large woman, and the figurative language in this passage isn't meant to draw attention to her size. Instead, the metaphor that turns Clarissa into a doomed mammoth standing "up to its knees in the tar" is being used to drive home a point about her age and mortality.
The mammoth in this passage is a fossil in the making. When the jackals come, it'll be killed and eaten, and its bones will sink into the tar and be preserved for who knows how many thousands of years. According to Willie Bass, Clarissa isn't too far off from being a fossil herself. Just as it does for every living person, her time will eventually run out.