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Flames glided in the river, small green flames, red flames, white flames, pursuing, overtaking, joining, crossing each other – then separating slowly or hastily. The traffic of the great city went on in the deepening night upon the sleepless river. (1.14)
The "traffic of the great city" seems somewhat hellish in its depiction through unnaturally colored flames. This contrasts sharply with the darkness of the river.
[Marlow]: "It was the farthest point of navigation and the culminating point of my experience. It seemed somehow to throw a kind of light on everything about me – and into my thoughts. It was somber enough too – and pitiful – not extraordinary in any way – not very clear either. No, not very clear. And yet it seemed to throw a kind of light." (1.15)
Marlow describes meeting Kurtz as an experience that "throw[s] a kind of light […] into my thoughts." Though readers may expect that light to help Marlow understand and clarify his experience, Marlow surprises us by saying that it was "not very clear." This makes the distinction between light and darkness again ambiguous.
"It [Africa] had ceased to be a blank space of delightful mystery – a white patch for a boy to dream gloriously over. It had become a place of darkness." (1.18)
Marlow associates the blank white spaces of a map with childhood innocence and a yearning to explore. As he grew up, the space darkened as the unknown was filled in – a reversal of the typical black representing unknown imagery.