=The Geological Formation.= The geological formation of the hills of Jerusalem also throws much light upon its history. The strata is inclined toward the southeast at an average angle of ten or twelve degrees. The surface rock is hard, silicious limestone with bands of flint. This crops out on the top of the temple mount, on the west side of the Kidron Valley, and in the quarries to the north of Jerusalem. Below is a bed of soft, white limestone, easy to cut and good for building, since it hardens with exposure. The presence of this underlying rock made possible the extensive system of cisterns and underground passages with which Jerusalem is honeycombed and which has played such a large part in the tragic history of the city. Beneath this layer is a hard, dolomitic limestone, white but streaked with pink. While the upper layers of rock are porous, this lower stratum holds the water, with the result that at the one point where it comes to the surface on the western side of the lower Kidron Valley there is a spring. The southern inclination of this hard rock also carries the drainage of the city to the south, to the point where the Kidron and Hinnom valleys meet. =The Water Supply.= Jerusalem, like most of the cities of Judah, was inadequately supplied with springs. The Virgin's Spring, which is in all probability identical with the ancient spring of Gihon, is intermittent. Its waters issue from a natural siphon in the rock, flowing according to the local rainfall, sometimes for only an hour or more, and at others for several days. It is entered by steps which lead into a natural cave, half way down the side of the Kidron Valley. From this point, in early times, the waters were conducted through the ridge of rock on which the city rested by a rock-cut tunnel, which is one of the wonders of ancient engineering. Thence they were carried to a point at the southern end of the Tyropœon Valley, where was once the famous Pool of Siloam, which was probably inside the ancient city walls. Here was built in later times a great basin, fifty-two feet square, from which the inhabitants of the city drew their chief supply of water.^{(112)} Still another conduit carried the overflow from this pool to the so-called Job's Well, four hundred and fifty yards to the southeast, where the Kidron and the Hinnom valleys meet. West of the temple area there was apparently in ancient times a pool which received the waters from the upper basin of the Tyropœon, but there is no clear geological or historical evidence of a living spring in the northern or western part of the city. Aside from their one perennial spring and these pools the inhabitants of ancient Jerusalem were wholly dependent upon rock-cisterns for their water supply. Modern excavations have revealed the remains of the aqueducts, by which the plentiful waters of the so-called Pools of Solomon, southwest of Bethlehem, were later conducted through the valleys and along the hillsides to Jerusalem.