Cynical, Stark, Poetic
Just to choose a totally random passage:
Black shapes crouched, lay, sat between the trees leaning against the trunks, clinging to the earth, half coming out, half effaced within the dim light, in all the attitudes of pain, abandonment, and despair. Another mine on the cliff went off, followed by a slight shudder of the soil under my feet. The work was going on. The work! (1.39)
Okay, fine, we didn't exactly choose this randomly. But Marlow's description of dying slaves is a good example of Heart of Darkness's tone. It's stark and unflinching, describing the dying slaves in really poetic language—all that "half coming out, half effaced" business—along with a kind of emotionless statement of facts: they slaves are dying, and the work is going on. Over and over, Conrad renders terrible scenes with a literary flick of the wrist. Maybe he sees poetry as the only way to deal with horror?
But notice the exclamation mark after the second "work." That's Marlow (or Conrad) being cynical: it tells us that he can't believe the work is just going on while all these humans are suffering, but it also tells us that he's not surprised. His journey into Africa has made him cynical about what humans are capable of.