Symbolism, Imagery, Allegory
This is an oft-repeated theme in the novel. Porthos is described most frequently as a "giant," a "Titan," and a "Hercules." Being crushed to death under an immense load of rocks is therefore an apt mode of death for Porthos. As he manages for a time to delay the inexorable downward weight of rock, his figure is reminiscent of the immortal Atlas in Greek mythology, who bears the weight of the world on his shoulders.