=9. The Great Use of Concrete.=--Going about Rome one is led to imagine, however, that many very pretentious structures are of solid brick. This is seldom the case. Bricks and tiles are often in evidence because they can be worked into the face of naturally ugly concrete to disguise the nakedness of its surfaces. _Concrete_ has really made it comparatively easy to create Rome as an enormous city. If concrete has not been invented by the Romans, they are at least the first great people to put it to a very general use. In their neighborhood can be found huge quantities of _pozzolana_,[5] a volcanic deposit which can be readily worked up into admirable cement. It is this very practical material which makes the vast domes, cupolas, and other architectural triumphs possible. Many a pretentious temple or residence flaunts a marble exterior; this, however, is a mere shell and covering; strip it away, and within is an enormous mass of concrete. This material can be handled by comparatively small labor gangs, rendering it feasible to erect huge structures without mobilizing such wholesale man-power as was needed for the great monuments of Egypt. It is very durable, almost nothing can destroy it. Indeed it will be written later that “This _pozzolana_ [for concrete] more than any other material contributed to make Rome the proverbial ‘Eternal City.’” [Middleton.] =10. Greek Architectural Forms Plus the Arch and Vault.=--Every building by the Tiber apparently bears the impress of Greece. Greek architects are said to have designed many of the finest public edifices, while Greek artists have chiseled the statues or painted the pictures which all the Roman world admires. The “orders” of the columns everywhere in evidence are the Doric, Ionic, or Corinthian that one might find at Athens, although it can be complained that the Romans are over-fond of the most ornate form--the florid Corinthian. [Illustration: TYPICAL TEMPLE FRONT.] In general, lovers of the purer architectural types of Hellas may allege that Roman architecture and ornamentation is too elaborate and extravagant. There are too many scrolls and floriated designs. Every possible surface is covered with statuary or bas-reliefs, often in decidedly inferior taste. There is too garish a display, also, of blue, green, white, and orange-colored marble. The whole effect of most Roman buildings is, therefore, _grand rather than beautiful_. It is the architecture of a civilization apparently growing a little weary and striving to startle itself by remarkable effects. [Illustration: ARCH OF CONSTANTINE: typical of many triumphal arches: date about 315 A.D.]