Marlow’s journey takes him up the Congo River. How does the river function as a metaphor for Marlow’s descent into Kurtz’s world?

At numerous points in the novel, Marlow notes the Company men’s obsession with ivory. Beyond its material value, what is the symbolic significance of ivory?

The novel is structured in such a way as to position Kurtz as Marlow’s ultimate destination. How does Kurtz slowly become the center point off the narrative?