How does Conrad create atmosphere and mood in "Heart of Darkness?"

Atmosphere and mood are highly subjective. For the
purposes of this answer, we will define Atmosphere as
sense of place and mood as sense of feeling.
Additionally, Heart of Darkness is about 40,000 words long,
so there are many, many examples throughout the text. One of each will be provided here,
which should inspire deeper
reading.

Atmosphere: For a
sense of place, the prose should be simple and direct. There is a
certain amount of license allowed for longer descriptions and metaphor; "purple prose"
is often seen in longer novels, but Conrad's prose is short and very
descriptive:

readability="11">

"I avoided a vast artificial hole somebody had
been digging on the slope, the purpose of which I found it impossible to divine. It
wasn't a quarry or a sandpit, anyhow. It was just a hole. It might have been connected
with the philanthropic desire of giving the criminals something to do. I don't know.
Then I nearly fell into a very narrow ravine, almost no more than a scar in the
hillside. I discovered that a lot of imported drainage-pipes for the settlement had been
tumbled in there. There wasn't one that was not broken. It was a wanton
smash-up."

The jumbled pipes
in a narrow ravine, the artificial hole with no purpose; each of these is simply
described and solidly realized in the description. No extra words, no glorified
metaphor, just a couple of holes that Marlow almost falls into. They serve no higher
purpose in the story but they establish the terrain he is traveling and the activities
that may have happened there. We don't need to know the composition of the dirt, or if
the pipes are corrugated or smooth; the simple description is all we need to visualize
the place completely.

Mood:
For a sense of feeling, the license for metaphor is greater. Less
significance is placed on description and more placed on the feel generated by the
words. A feeling can be created using phrases that have little meaning in the larger
text:

"The
smell of mud, of primeval mud, by Jove! was in my nostrils, the high stillness of
primeval forest was before my eyes; there were shiny patches on the black creek. The
moon had spread over everything a thin layer of silver -- over the rank grass, over the
mud, upon the wall of matted vegetation standing higher than the wall of a temple, over
the great river I could see through a sombre gap glittering, glittering, as it flowed
broadly by without a murmur. All this was great, expectant, mute, while the man jabbered
about himself."

A jungle,
thick and unfeeling, drenched with mold and moonlight,  yet completely silent while the
bricklayer talks. His words echo in the night, even reflecting off the silver moonlit
mud, and fill the silence as easily and uselessly as crickets or night-birds. This is
purely a sense of feeling, of words emptying in a cold impassive jungle, by a river that
flows eternally without care. Is the mud important in the text? Will it be a key part of
the climax? Does it matter when we have a sense of a bleak, entirely inhumane world
which simply doesn't care about a bricklayer's life story?