In the inner room Pausanias saw the trident-mark, naturally near the φρέαρ. The first produced the second, according to Apollodorus, III, 14, 2. Pausanias did not see them πρὸ τῆς ἐσόδου but ἔνδον. There is no authority whatever for identifying the marks in the rock beneath the north porch with those made by the trident of Poseidon, except common consent in recent times. If the trident-mark lay within the Erechtheum what deity made that outside, and beneath the porch, a mark which was beyond question an object of cult? "Die Stelle welche Zeus mit seinem Blitze getroffen hatte, wurde mit einem Puteal umgeben und blieb unter freiem Himmel" (Dörpfeld, _Ath. Mitt._, 1903, p. 467). An altar of Zeus Hypatus stood before the entrance. The coincidence of place πρὸ τῆς ἐσόδου and ἐν τῇ προστάσει τῇ πρὸς τοῦ θυρώματος where, according to the official inscription the altar of the Thyechous stood, outweighs any objection to the identification of the two altars based on difference of name in the two records, ὁ βωμὸς τοῦ θυηχοῦ and Διὸς βωμὸς Ὑπάτου. Pausanias departs from the official terminology of building inscriptions. The rotunda at Epidaurus was called in the building inscription θυμέλη (cf. Cavvadias, Τὸ Ἱερὸν τοῦ Ἀσκληπιοῦ ἐν Ἐπιδαύρῳ, p. 50). Pausanias called it θόλος. The official name for the Erechtheum does not occur in literature nor in inscriptions except in the report of the commissioners. It is not surprising then if Pausanias failed to call the altar ὁ βωμὸς τοῦ θυηχοῦ. This name gives not the slightest clue to the god to whom it was erected. The suggestion of Michaelis (_Jb. Arch. Inst._, 1902, p. 17) that the altar may have been one to Poseidon proceeds from the logical idea to make it that of the god who is thought to have made the marks in the rock beneath the porch.
LOOKING NORTH IN THE LINE OF THE EASTERN INTERIOR CROSS-WALL. A VIEW SHOWING THE ORTHOSTATE WHICH WAS IN CONTACT WITH THE INTERIOR WALL AND THE ROUGH SURFACE (X) OF THE NATIVE ROCK IN THE LINE OF THE LATTER]
The altar in the north porch was one to Zeus and its presence there suggests the reasonable theory that the marks in the rock below it and the square hole in the roof above are a memorial of the thunderbolt which he hurled at Erechtheus according to Hyginus (_Fab._, 46). _Cf._ Petersen, _op. cit._, p. 72. One cannot say which is the earlier tradition, that preserved in Hyginus or that in Euripides (_Ion_, 281) according to which πληγαὶ τριαίνης thrust Erechtheus into a χάσμα χθονός (Furtwängler, _Masterpieces_, p. 436, note 3). There was a tradition that Zeus, at the request of Poseidon, killed Erechtheus with a thunderbolt, a tradition which becomes the more interesting in the light of an inscription found on the Acropolis (Lolling, Δελ. Ἀρχ., 1890, p. 144) which proves that an ἄβατον Διὸς Καταιβάτου existed there. The stone bearing the inscription was found in a mediaeval wall north of the northeast corner of the Parthenon. Three surfaces of the fragment are preserved showing that it came from a corner perhaps of a low wall enclosing the ἄβατον. One side of the block which is Pentelic marble is finely polished. There are no dowel or clamp-holes preserved and it is impossible to recover the dimensions of the original block. The face which bears the inscription of the late fourth century seems to have been redressed, since chisel marks are evident. The inscription may then have been recut. It is tentatively suggested that this fragment was part of the curb about the opening in the floor of the north porch.