On the plans the Insulae are designated by Arabic numerals, but in the text small Roman numerals are used for the sake of clearness; thus, Ins. IX. i. 26, means the first Insula of Region IX, No. 26. The names of several of the more important streets, as of the better known houses, are given in the text in the English form. CHAPTER VI _BUILDING MATERIALS, CONSTRUCTION, AND ARCHITECTURAL PERIODS_ Six centuries lie between the dates of the earliest and the latest buildings at Pompeii; and in order to understand any structure rightly we must first of all ascertain to what period it belongs. It is indeed rarely possible to fix dates with exactness for the earlier time; but certain periods are so clearly differentiated from one another, that in most cases there is no room for doubt to which of them a building is to be assigned. Before undertaking to characterize these periods, however, it will be necessary briefly to notice what building materials were used, and how they were turned to account in construction. Exclusive of wood, which was more freely used in Pompeii than in Campanian towns to-day, the principal building materials were Sarno limestone, two kinds of tufa (gray and yellow), lava, a whitish limestone often called travertine wrongly, marble, and brick. The Sarno limestone (_pietra di Sarno_) is a deposit from the water of the Sarno, and is found in beds along the course of the river. It contains many impressions of the leaves and stems of plants, and varies greatly in compactness; it closely resembles the Roman travertine, except that it has a more decided yellowish tint. Gray tufa is a volcanic dust which has been hardened by the presence of water into rock. It has a fine grain, and is easily worked; it was quarried in the vicinity of Nocera. The volcanic dust which formed the yellow tufa was thrown out in an earlier period, when the Sarno plain was still a part of the sea, and so hardened in salt water; it is more friable than the gray tufa, and not so durable. The lava, which came originally from Vesuvius, was quarried at Pompeii. Three varieties may be distinguished, differing in density according as they were taken from the lower or the upper strata: solid lava, or basalt, which, being heavy and extremely hard, was extensively used for pavements and thresholds; slag, like the scoriae found on the sides of Vesuvius to-day; and cruma, the foam of the lava stream, which is light and porous, but on account of its hardness has good resisting qualities.