γύαλον, ‘hollow’ ( O. C. 1491 ff., n.), is here properly the chamber itself, while κοίλας πέτρας (possessive gen.) is the cavernous rock which contains it. Cp. Helen. 189 “πέτρινα μύχατα” | “γύαλα”, ‘inmost recesses of the rocks.’ θερμὸν καὶ παγετῶδες. Contrast this with the description by Odysseus, 17 ff. Cp. Hes. Op. 640“Ἄσκρη, χεῖμα κακῇ, θέρει ἀργαλέῃ, οὐδέ ποτ᾽ ἐσθλῇ”.