Quote 148
"The idleness of a passenger, my isolation amongst all these men with whom I had no point of contact, the oily and languid sea, the uniform sombreness of the coast, seemed to keep me away from the truth of things, within the toil of a mournful and senseless delusion. The voice of the surf heard now and then was a positive pleasure, like the speech of a brother. It was something natural, that had its reason, that had a meaning." (1.30)
Marlow's isolation from other men and the unchanging scenery of the coast lulls him into a comforting and false sense of security. In retrospect, he knows that he was living a "senseless delusion" in which nature is "a positive pleasure" and even makes sense. The further he gets into the interior, the more he becomes disillusioned.
Quote 149
"There were moments when one's past came back to one, as it will sometimes when you have not a moment to spare for yourself; but it came in the shape of an unrestful and noisy dream, remembered with wonder amongst the overwhelming realities of this strange world of plants, and water, and silence. And this stillness of life did not in the least resemble a peace. It was the stillness of an implacable force brooding over an inscrutable intention. It looked at you with a vengeful aspect." (2.5)
Marlow's very own comfy memories start to seem just as alien as the wilderness. This surrealism makes Marlow feel as though the jungle around them is alive and looking at him "with a vengeful aspect," and—as we all know—thinking that the world is out to get you is a pretty good sign of madness. (Usually. Unless there's a good reason they're after you.)
Quote 150
"The steamer toiled along slowly on the edge of a black and incomprehensible frenzy. The prehistoric man was cursing us, praying to us, welcoming us—who could tell? We were cut off from the comprehension of our surroundings; we glided past like phantoms, wondering and secretly appalled, as sane men would be before an enthusiastic outbreak in a madhouse. We could not understand because we were too far and could not remember because we were travelling in the night of first ages, of those ages that are gone, leaving hardly a sign—and no memories." (2.7)
Catching sight of wild native Africans in their homeland rouses fear in the pilgrims. They feel as if they have traveled to a place where nothing is comprehensible. They cannot read the attitude of the Africans towards them. Marlow compares their mental state to that of inmates in an insane asylum right before an outbreak—teetering on the edge of insanity.