Final answer:
The extended metaphor in the poem 'Slow through the Dark' is the brook symbolizing the journey of life.
Step-by-step explanation:
The extended metaphor in the poem 'Slow through the Dark' is the comparison between the brook and life itself. The poet describes the brook 'putting out of the roots of the old tree and flowing to the meadow', which symbolizes the journey of life from its origin to its destination. The image of the brook flowing represents the passage of time and the constant movement of life.