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Finally we found the place where the mummies were, and we went in.
"You know how the Egyptians buried their dead?" I asked the one kid. "Naa."
"Well, you should. It's very interesting. They wrapped their faces up in these cloths that were treated with some secret chemical. That way they could be buried in their tombs for thousands of years and their faces wouldn't rot or anything. Nobody knows how to do it except the Egyptians. Even modern science." (25.32-35)
The mummies don't worry Holden the way the ducks do. While he's concerned over the ducks disappearing, he's OK with the mummies, because they can be preserved. This adds a new spin to the death vs. disappearing conversation. Perhaps Holden distinguishes between the two, and finds the latter far more disconcerting. This would mean that, the case of Allie, it's his burial under the cemetery (having "disappeared," so to speak) that Holden finds most disturbing.