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Hunters for gold or pursuers of fame, they all had gone out on that stream, bearing the sword, and often the torch, messengers of the might within the land, bearers of a spark from the sacred fire. What greatness had not floated on the ebb of that river into the mystery of an unknown earth!…The dreams of men, the seed of commonwealths, the germs of empires. (1.6)
The colonists are described as shiny, altruistic pioneers sallying forth into the dark uncivilized world to bring salvation and civilization to the ignorant races. Oh, but also terror, rape, enslavement, and forced conversion. Awesome!
[Marlow]: "Mind, none of us would feel exactly like this. What saves us is efficiency—the devotion to efficiency. But these chaps were not much account, really. They were no colonists; their administration was merely a squeeze, and nothing more, I suspect. They were conquerors, and for that you want only brute force—nothing to boast of, when you have it, since your strength is just an accident arising from the weakness of others. They grabbed what they could get for the sake of what was to be got. It was just robbery with violence, aggravated murder on a great scale, and men going at it blind—as is very proper for those who tackle a darkness. The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much. What redeems it's the idea only. An idea at the back of it; not a sentimental pretence but an idea; and an unselfish belief in the idea." (1.13)
Aaaand, now Marlow undermines everything he just said about the nobility and good intentions of the explorers. He's seen how messed up colonization really is, and he knows that the colonizing countries care only about efficiency and profit. The explorers aren't heroes; they're robbers and murderers who just wanted to bring home profit.
"I had then, as you remember, just returned to London after a lot of Indian Ocean, Pacific, China Seas - a regular dose of the East - six years or so, and I was loafing about, hindering you fellows in your work and invading your homes, just as though I had got a heavenly mission to civilize you." (1.16)
Marlow makes fun of the colonist's motto—to civilize savages—by comparing it to an idle traveler imposing himself on hosts too generous to make him leave. The implication is that the colonists' arrival with all their rhetoric of civilization is ultimately undesired by the native African inhabitants. What, you mean the native Africans don't actually want to be civilized?