The “thunder-strokes” refer to “Titan heaven’s first-born, With his enormous brood” (Par. Lost, I. 510).  Cf. Virgil, Æn. vi. 580:— “Hic genus antiquum Terræ, Titania pubes, Fulmine dejecti fundo volvuntur in imo.” Ibid. v. 585:— “Vidi et crudeles dantem Salmonea pœnas, Dum flammas Jovis et sonitus imitatur Olympi.” Clem. Alex. (Exhort. II. § 37):—“Æsculapius lies struck with lightning in the regions of Cynosuris.”  Cf. Virg. Æn. vii. 770 ss.