This snippet is fundamentally a tour. But it has a particular structure:
- The intrusion: Dickens introduces an intrusion into the envrionment. In this case, it's fog.
- Poles 1 & 2: The intrusion appears at two opposite ends of the environment.
- Poles 3 & 4: The intrusion appears in two more opposite ends, along a different axis.
- Things: We follow the intrusion as it relates to a series of things.
- People: We follow the intrusion as it relates to a series of people.
So you can see that this visual tour guide (the fog) and the tour follows a kind of progressive logic (west-east, north-south, things, people).