the intention of the analysis was to determine the most likely provenance of the scoria samples taken from the caementa of vaults of five buildings in Rome dating from the mid-first century b.c. to the late third century a.d. The samples are visually very similar to the scoria used in the dome of the Pantheon, which was determined by Gioacchino De Angelis d'Ossat in 1930 to have been a product of Vesuvius. Because there has recently been some suggestion that a similar looking material produced by the Colli Albani system just south of Rome also may have been used for vaulting in Rome, this analysis is designed to determine whether the material from Vesuvius continued to be imported over a long period or whether it was replaced by a local but similar-looking material. In his later study of the lightweight material from the “Temple of Minerva Medica,” De Angelis d'Ossat found pumice produced by the Sabatini system north of Rome. In the present study, petrographical analyses of thin sections were used to identify the crystal fragments within each sample, and then the resulting mineralogical profile was compared to the compositional data for volcanic deposits from the three most likely volcanic districts to have supplied the Roman builders: Vesuvius, Colli Albani, and Sabatini.

Because the color of the scoria varies from dark brown to reddish brown (Pl. VIII, IX), samples from each end of the spectrum were included.