Daylight is a glimmer "...in a cavern of night..." (Gallicenae, p. 153), which is appropriate since Mithras is worshiped in a cave.
At nightfall, when Gratillonius and his visiting father return to the palace, wind yells, driving rain and chill down the streets, and:
"Under the sea wall, tide ramped and snarled." (p. 158)
Why does Chapter VII end with this sentence? Because the Mithraeum is below the waterline and vulnerable to the sea god, Lir, Who will overwhelm His city, inundating everything in it, especially the basements built just beneath the sea wall. The doom that is to come is foreshadowed in much of the preceding text.