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THE ANCIENT CITY] stratified form, with frequent cavities and fissures lined with crystals. As Vitruvius (ii. 5) says, it is a good weather-stone, but is soon calcined by fire. If laid horizontally it is very strong, but if set on end its crystalline structure is a great source of weakness, and it splits from end to end. Neglect on the part of Roman builders of this important precaution in many cases caused a complete failure in the structure. This was notably the case in the rostra. (6) Pulvis Puteolanus (pozzolana), so called from extensive beds of it at Puteoli—a volcanic product, which looks like red sandy earth, and lies in enormous beds under and round the city of Rome. When mixed with lime it forms a very strong hydraulic cement, of equal use in concrete, mortar or undercoats of stucco. It is to this material that the concrete walls of Rome owe their enormous strength and durability, in many cases far exceeding those of the most massive stone masonry. Vitruvius devotes a chapter (bk. ii. ch. 6) to this very important material.

Bricks were either sun=dried (lateres crudi) or kiln-baked (lateres cocti, testae). The remarks of Vitruvius (ii. 3) seem to refer wholly to sun-dried bricks, of which no examples now exist in Rome. It is important to recognize the fact that among the existing ancient buildings of Rome there is no such thing as a brick wall or a brick arch in the true sense of the word; bricks were merely used as a facing to concrete walls and arches and have no constructional importance. Concrete (opus caementicium, Vitr. ii. 4, 6, 8), the most important of all the materials used, is made of rough pieces of stone, or of fragments of marble, brick, &c., averaging from about the size of a man's fist and embedded in cement made of lime and pozzolana-forming one solid mass of enormous strength and coherence. Stucco, cement and mortar (tectorium, opus albarium and other names) are of many kinds; the ancient Romans especially excelled in their manufacture. The cement used for lining the channels of aqueducts (opus signinum) was made of lime mixed with pounded brick or potsherds and pozzolana; the same mixture was used for doors under the “nucleus” or finer cement on which the mosaic or marble paving-slabs were bedded, and was called caementum ex testis tunsis. For walls, three, or four coats of stucco were used, often as much as 5 in. thick altogether; the lower coats were of lime and pozzolana, the finishing coats of powdered white marble (opus albarium) suitable to receive painting. Even marble buildings were usually coated with a thin layer of this fine white stucco, nearly as hard and durable as the marble itself—a practice also employed in the finest buildings of the Greeks—probably because it formed a more absorbent ground for coloured decoration; stone columns coated in this way were called “columnae dealbatae” (Cic. In Verr. ii. 1, 52 seq.). For the kinds of sand used in mortar and stucco, Vitruvius (ii. 4) mentions sea, pit and river sand, saying that pit sand is to be preferred.

From the latter part of the 1st century hard stones—granites and basalts—were introduced in great quantities. The

basalts—“basanites” of Pliny (xxxvi. 58)—are very refractory, and can only be worked by the help of emery or diamond dust. The former was obtained largely at Naxos; diamond-dust drills are mentioned by Pliny (H.N. xxxvii. 200). The basalts are black, green and brown, and are usually free from spots or markings; examples of all three exist, but are comparatively rare. The red variety called “porphyry” was used in enormous quantities. It is the “porphyrites” of Pliny (H.N.