In the novel, Heart of Darkness, what is the Chapman Lighthouse and what does it represent in the story?

In Chapter One of "The Heart of Darkness" , the narrator refers to he Chapman Lighthouse as "a three-legged thing erect on a mud flat, shone strongly". This lighthouse actually existed at the end of the Thames River. It was demolished in 1957 because it falling apart. As one of the last things a sailor could see when leaving the river, it's light represents the light of civilization. However, in context, the fact that the lighthouse is built on mud-flats reinforces Marlow's next statement that “And this also...has been one of the dark places of the earth.” Conrad is beginning his symbolic contrast between light and darkness, civilization and savagery that will continue throughout the novel.