In "Composed upon Westminster Bridge," what is the significance of the exclamation at the end of the poem?

Certainly the ending of the poem is remarkable for the
exclamation that seems to escape the mouth of the speaker
involuntarily:

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Dear God! the very houses seem
asleep;

And all that mighty heart is lying
still!

Note how these final
lines are significant because they emphasise what seems to impress the speaker most
about this view of London: the city's calmness and tranquillity. In these lines,
Wordsworth uses personification to compare the houses to sleeping creatures, emphasising
the lack of movement and peacefulness of the view. Let us not ignore either that the
poem ends on a paradox. A heart can't be both alive and still at the same time, and yet
the "mighty heart" of London, that throbs with such action and movement at the best of
times, from the speaker's vantage point appears to be "still," emphasising the tranquil
and peaceful mood that dominates the poem.