The rock strata at Jerusalem fall with an inclination of about ten degrees south-east from the watershed, so that the rain-water is carried naturally in this direction towards the junction (below Siloam) of the Kidron, the Tyropœon and the Hinnom valleys. The town indeed has the appearance of sliding downhill towards the south-east, the Ophel spur being the lowest of those covered by the city at its time of greatest magnitude, when Jerusalem--including the 30 acres of the Temple enclosure--covered about 300 acres in all, being half as large again as the present city within the Turkish walls. The lowest rock stratum, which appears in the low cliffs on the east side of the Kidron, is a hard dolomitic limestone, impervious and forming the bed for streams which sink through the more porous upper limestone. It appears again on the watershed to the north-west, and is known as the Santa Croce marble, being mottled with red, which--on the hillock of the traditional Calvary--was regarded as being due to the blood of Christ. This formation is of the Greensand period geologically, and the stone is known as _mezzeh_, or “superior,” in Arabic. Above it lie beds of fine but rather soft building stone, belonging to the Lower Chalk age, and called in Arabic _meleki_, or “royal” stone.[83] In this white limestone the Temple cisterns are cut. Another stratum of hard limestone, or _mezzeh_, lies over the _meleki_, and above this on Olivet is the white Upper Chalk, full of ammonites, hippurites, and other characteristic shells, with beds of the Eocene age, including a capping of nummulitic limestone. These porous strata are known as _k’akûli_, or “conglomerate,” and _nâri_, or “fire stone.”