LATIUM, the ancient name of the portion of central Italy, bounded on the north-west by Etruria, on the south-west by the Tyrrhenian sea, on the south-east by Campania, on the east by Samnium and on the north-east by the mountainous district in habited by the Sabini, Aequi and Marsi. Latium, originally the land of the Latini, a tract of limited extent, was, after the over throw of the Latin confederacy, when the neighbouring tribes of the Rutuli, Hernici, Volsci and Aurunci were also reduced to the condition of subjects and citizens of Rome, extended to com prise Lhem all. It thus denoted the whole country from the Tiber to the mouth of the Savo, and just included the Mons Massicus.
This change was not formally established till Augustus formed of this larger Latium, taken together with Campania, the first region of Italy : but it is already recognized by Strabo as well as by Pliny, who terms the additional territory thus incorporated Latium Adjectum, designating the original Latium from the Tiber to Circeii as Latium Antiquum.
This consisted principally of an exten sive plain now known as the Campagna di Roma, in the centre of which rise the volcanic Alban hills, hounded towards the interior by the Apennines, which rise very abruptly from it to a height of 4,000-5,00o feet. Several of the Latin cities, includ ing Tiber and Praeneste, were situated on the terrace-like under falls of these mountains, while Cora, Norba and Setia were placed in like manner on the slopes of the Volscian mountains (Monti Lepini), a rugged and lofty limestone range, which runs parallel to the main mass of the Apennines, being separated from them, however, by the valley of the Trerus (Sacco), and forms a con tinuous barrier from there to Terracina. No volcanic eruptions are known to have taken place in the Alban hills within the his toric period, though Livy sometimes speaks of it "raining stones in the Alban hills" (i. 31, xxxv. 9—on the latter occasion it even did so on the Aventine). It has been recently ascertained, too, that the earliest tombs of the necropolis of Alba Longa (q.v.) do not, as has been asserted, lie beneath a stratum of peperino.
Earthquakes (not of a violent character within recent centuries, though the ruin of the Colosseum, and many other buildings of ancient Rome is certainly to be ascribed to this cause, and most probably to an earthquake in A.D. 854) are not unknown even at
the present day in Rome and in the Alban hills, and there is a seismographic observatory at Rocca di Papa. The surface is by no means a uniform plain, but is a broad undulating tract, fur rowed throughout by numerous depressions, with precipitous banks, serving as water-courses, though rarely traversed by any considerable stream. As the general level of the plain rises, these channels by degrees assume the character of ravines.
The hills on the right bank of the Tiber culminat ing in Monte Mario (455 ft.) belong to the Pliocene; they con sist of a lower bluish-grey clay and an upper group of yellow sands and gravels. This clay since Roman times has supplied the ma terial for brick-making, and the valleys which now separate the summits (Janiculum, Vatican, Monte Mario) are partly artificial. On the left bank this clay has been reached at a lower level, at the foot of the Pincian hill, while in the Campagna it extends below the later volcanic formations. The earlier eruptions oc curred at the bottom of the Pliocene sea, and the tufa, which extends over the whole Campagna to a thickness of 30o ft. or more, was formed. At the same time, hot springs, containing abun dant carbonate of lime in solution, produced deposits of traver tine. Later, after the Campagna, by a great general uplift, had become a land surface, volcanic energy found an outlet in com paratively few large craters, which emitted streams of hard lava as well as fragmentary materials, the latter forming sperone (lapis Gabinus) and peperino (lapis Albanus), while upon one of the former, which runs from the Alban hills to within 2 m. of Rome, the Via Appia was carried. The two main areas near Rome are formed by the group of craters on the north (Bracciano, Bolsena, etc.) and the Alban hills on the south, the latter consisting of one great crater with a base about 12 m. in diameter, in the centre of which a smaller crater was later on built up (the basin is now
as the Campo di Annibale) with several lateral vents (the Lake of Albano, the Lake of Nemi, etc.). The Alban Mount (Monte Cavo) is almost the highest point on the rim of the inner crater, while Mount Algidus and Tusculum are on the outer ring wall of the larger (earlier) crater.