She parted from Simon as the cavalcade left the Park, and will see it pass from Fownes street, four blocks south of here, a mile and a half (15min?) from the Park, having crossed the river and bought a book. So maybe this scene is later???
in Merchant's arch?? When Lenehan sees Bloom, the cavalcade hasn't yet left the Park, so Dilly is still with Simon (so if she crossed the river by the Halfpenny bridge and visited that bookcart, Bloom should have been long gone)