in "Burbank with a Baedeker: Bleistein with a Cigar":
The horses, under the axletree
in "Burnt Norton," Four Quartets.
Garlic and sapphires in the mud
The axletree a particular kind of tree that was used in classical times for the axis of a chariot - 'axis' itself thus bearing testimony to its origin' - and in classical literature is frequently used as a pars pro toto for Phoebus' charoit that draws the sun across the sky, and from that the phrase refers to the sun itself: cf. the words with which Phoebus himself warns his son, in Ovid's Metamorphoses: "There is not one of all the Gods that dares / (However skill'd in other great affairs) / To mount the burning axle-tree, but I."
In "Burbank" the axletree has the familiar interpretation of the chariot of the sun, drawn through the sky by horses. "under the axletree" would then be interpreted as hinged up to the chariot. Istria is where the sun comes up from the perspective of Venice, and hence the imagery that the horses beat up the dawn in Istria.
Another possible interpretation - and they are not mutually exclusive - is that the horses are approaching literally from Istria, with the axletree, the sun, rising overhead. In the context of the evocation of Antony and Cleopatra in these lines, the horses may then be those of Ceasar's approaching army; and translated another of the poem's layers, the horses may also be those of the approaching army of Sir Ferdinand Klein, whom seems to replace Burbank in the seventh stanza. This would make sense if we take Sir Ferdinand to represent Austria, which was the first nation to defeat and annex Venice fully, and his approach from Istria would not only phonetically of Austria, but also from a historical perspective; Istria belonged to Venice before it was taken over.
Southam writes in relation to imagery in Gerontion, in the following lines:
According to Southam, Chapman is here "drawing upon the classical tradition that sinners were punished by being sent into an eccentric, outward orbit which would carry them away into space. The bear (usually known as the Great Bear) is a constellation in the Northern Hemisphere."
In "Burnt Norton" the imagery is more complex, a culmination perhaps of the ideas used in both "Gerontion" and "Burbank":
Garlic and sapphires in the mud
Chapman's image here converges with the crash that resulted from Phoebus's over-ambitous son - the bedded axletree converges with the imagery Ovid uses for the murder of the first of Niobe's sons, who is riding his chariot outside the city-walls when he is struck by the first of Apollo's arrows, who is punishing Niobe for her pride. This image is of mankind's pride and ambition which leads to war, and thus combined with Chapman's image of the sinners being sent into space as punishment, which we recognise in our moving away from earth into space, past the sun (the moving tree), into the realm of the stars and beyond.
I think the first reference Eliot makes to the axletree outside of his poetry is during the time when "Burbank" was first published, in an article he wrote for The Egoist (July 1919). In the essay, Eliot describes the intimate relationship a poet can have with a predecessor, summarised roughly in his line "We do not imitate, we are changed; and our work is the work of a changed man; we have not borrowed, we have been quickened, and we become bearers of a tradition."
The axletree appears in the example Eliot gives to illustrate how such a process may affect a writer, as well as esthetic appreciation of him (therewith of course implicitly defending his own style, of which perhaps "Burbank" is a culmination). Eliot compares this section in Chapman Bussy D'Ambois (Act V, scene 4, lines 104-6) with Seneca Hercules Furens (p.53) on at least three separate occasions, among which the essay in the July 1919 edition of The Egoist (quoted below) and "The Use of Poetry and the Use of Criticism" in 1933 (see Southam, in overview of the criticism).
Hercules brings Smith to Chapman's Bussy D'Ambois (Act V, scene 4, lines 104-6), one of Eliot's favourite passages which he has compared three times with a passage from Seneca's Hercules Furens (p.53) and "of course the horses are identifiable both with those of the solar chariot and with those of Saint Mark's cathedral at Venice.
Moody notes this too, and adds that "the beat of their even feet is that of Horace's oncoming Death - 'Time's winged Chariot hurrying near', as Marvell rendered it."
Southam (1994) refers to his note to Gerontion, thus linking the imagery here with the imagery in that poem, in which the integration of George Chapman into Eliot is more extensive. In that note, Southam points out that Eliot himself, in 1933, explained the imagery in Gerontion's lines, which derive from Bussy's dying speech:
Fly where the evening from the Iberian vales