In chapter 27, the three men find themselves walking through an unusual, wonderful forest. The trees are the same "common humble trees of Mother Earth," but they are much, much bigger. The plants are likewise huge. The narrator then starts to wonder how big the animals who once lived in this place must have been. The narrator's uncle invites the narrator to "look at the dust [they] are treading under foot."
The narrator observes that the ground is covered with "the bones of antediluvian animals," meaning animals that existed before the biblical flood. The fact that many of the bones seem to have degraded to "dust" emphasizes how old they must be, which in turn suggests that these bones are not indicative of recent deaths. If the bones were a sign of recent deaths, the atmosphere would perhaps be gruesome. However, because they are a sign of deaths which occurred many thousands or even millions of years ago, they are not gruesome, but instead fascinating.
The narrator picks up a few bones, which in some cases are "as big as the trunks of trees." He speculates that one bone might be "the lower jawbone of a mastodon," and another the leg bone from a "Megatherium." The men examining these bones are not afraid, but rather interested. These bones are not gruesome because they are, first and foremost, interesting fossils that allow the characters to better imagine a different world and a different time. For the same reason, if one were to walk into a natural history museum and see the bones of long-dead dinosaurs, one would probably find these bones more interesting than gruesome.