Please explain the following lines from "A Refusal to Mourn the Death, by Fire, of a Child in London."Deep with the first dead lies London's...

The central point of this poem is the author's radical
reassessment of death given his belief that when we die we actually rejoin nature, which
is an incredibly powerful force of regeneration and life in the universe. Thus, the
speaker says, he deliberately eschews the forms of mourning that humans adopt because
they fail to recognise the way that life metamorphosises and continues rather than
ceases to be.

In the final stanza, which is the section of
the poem that you are asking about, the dead girl that the poem's title refers to is
said to join life in her death, becoming reunited with the "long friends" who have
passed away before her and the "grains beyond age," which is a symbol of the sands of
time and seeds, and finally the "dark veins of her mother," which refers to the earth
and the way that the veins of the earth are its rivers. What is interesting is that
nature itself is depicted as "unmourning" in the form of the river Thames, even though
the girl is called "London's daughter." This is because, given the view of death and
nature, the Thames is regarded as the river of regeneration, and is teeming with life as
the cells of "London's daughter" form a part once again of the city. There is a pleasing
symmetry in the way that rivers, journeying from their source, then flow into the sea,
all being part of the same matter, just as humans, when they die, become part of the
bigger life of nature and the universe, as it is all part of the same thing as
well.

The final line of the poem cements the main argument
of the speaker. Whilst individual life forms can die, we rejoin nature, which is a
scientific force that is built to endlessly regenerate. Because of this, our first death
will be our last, because when we rejoin nature, we can no longer "die" in the way that
humans can.