“The word ‘ivory’ rang in the air, was whispered, was sighed. You would think they were praying to it. A taint of imbecile rapacity blew through it all, like a whiff from some corpse. By Jove! I’ve never seen anything so unreal in my life. And outside, the silent wilderness surrounding this cleared speck on the earth struck me as something great and invincible, like evil or truth, waiting patiently for the passing away of this fantastic invasion.”

This passage explains the importance of ivory during the time in the story. This shows that ivory has become so important to the natives lives that it has become a sacred item to them. So much so that it seems that they might worship it.

“The horror! The horror!”

This passage contains the final words of Kurtz just before he dies. Thus ending the legacy of what was once a great man.

“In a few days the Eldorado Expedition went into the patient wilderness, that closed upon it as the sea closes over a diver. Long afterwards the news came that all the donkeys were dead. I know nothing as to the fate of the less valuable animals. They, no doubt, like the rest of us, found what they deserved. I did not inquire.”

This passage contains Marlow's view on the Eldorado Expedition party. In this passage he shows his dislike of them by calling the people of the group "the less valuable animals" versus the donkeys. This passage shows that his care for others has dived to basically nothing and that his focus on the task at hand is strong. 

“His very existence was improbable, inexplicable, and altogether bewildering. He was an insoluble problem. It was inconceivable how he had existed, how he had succeeded in getting so far, how he had managed to remain -- why he did not instantly disappear.”

This passage states Marlow's shock and awe on the fact that Kurtz has survived this long on his own. 

“It was unearthly, and the men were—No, they were not inhuman. Well, you know, that was the worst of it—the suspicion of their not being inhuman. It would come slowly to one. They howled and leaped, and spun, and made horrid faces; but what thrilled you was just the thought of their humanity—like yours—the thought of your remote kinship with this wild and passionate uproar. Ugly. Yes, it was ugly enough; but if you were man enough you would admit to yourself that there was in you just the faintest trace of a response to the terrible frankness of that noise, a dim suspicion of there being a meaning in it which you—you so remote from the night of first ages—could comprehend. And why not?”

In this passage Marlow ponders on the fact that he is of the same species as the natives of this country. This wouldn't be as big a deal for him if it weren't for the fact that the natives of Africa are deemed "savages" and/or insignificant throughout the novel.

“But his soul was mad. Being alone in the wilderness, it had looked within itself and, by heavens I tell you, it had gone mad.”

This passage speaks of Kurtz's mental state. Kurtz had become insane from being in the Congo wilderness for so long with only the natives for company.

“The brown current ran swiftly out of the heart of darkness, bearing us down towards the sea with twice the speed of our upward progress; and Kurtz’s life was running swiftly, too, ebbing, ebbing out of his heart into the sea of inexorable time. . . . I saw the time approaching when I would be left alone of the party of ‘unsound method.’”

In this passage Marlow speaks of his leave on the steam boat away from the Inner Station with Kurtz and his ivory load on hand. Marlow speaks of the river, even though its current is brown, as the beacon of light back to civilization from the darkness that is the uncivilized Congo.

“We penetrated deeper and deeper into the heart of darkness”

This passage is taking place during the traveling on foot between stations by Marlow and his group. This was significant because it is in this passage that Marlow refers to the Congo as "the heart of darkness"

“I was within a hair’s-breadth of the last opportunity for pronouncement, and I found with humiliation that probably I would have nothing to say. This is the reason why I affirm that Kurtz was a remarkable man. He had something to say. He said it. . . . He had summed up—he had judged. ‘The horror!’ He was a remarkable man.”

This passage explains the impact of Kurtz's death on Marlow. While Marlow couldn't say much during his near fatal sickness earlier on in the story, Kurtz is able to say many things and his final words are words that may have multiple meanings but the exact meaning will never be found out. Thus causing Kurtz to be stuck in Marlow's head for probably the rest of his life, wondering what Kurtz had referred to just before he died.

“Your strength is just an accident owed to the weakness of others.”

This passage seemed more of the author speaking rather than his character. Joseph Conrad seems to be criticizing the Belgian monarchy on their rise to power through the despicable use of horribly cheap labor and the mistreatment of millions of people during the time.