On the stairs of the Capitol are also placed the marble sculptures called the trophies of Marius which were brought from the ruin on the Campus Esquilinus, not very far from the Arch of Gallienus. At the top of the steps stand two equestrian figures of the Dioscuri, said to have come from the neighbourhood of the Theatre of Balbus, and the statues of Constantine and his son Constans from the Baths of Constantine on the Quirinal. At the foot of the staircase are the two Egyptian lions found as above mentioned near the Church of S. Stefano in Cacco. The history of the bronze equestrian statue of M. Aurelius, now in the Piazza of the Capitol, cannot be traced as Palladio states to the Temple of Antoninus and Faustina, but it was more probably found as Fea has recorded, near the Arch of Septimius Severus. 2. A bronze cista mistica found at Præneste is in the Kircherian Museum. 3. The wooden beams from the Villa of Cæsar in the lake of Nemi, are kept partly in the Kircherian Museum, and partly in the Gallery on the right hand of the Vatican Library. 4. The caricature of Alexamenos from the Palatine is in the Kircherian Museum. 5. The mosaics from the Baths of Caracalla are in the Hall of Mosaics at the Lateran. 6. One of the white marble columns from the Basilica of Constantine is placed in front of the Basilica of S. Maria Maggiore. 7. The history of the great pair of figures on the Piazza del Quirinale called the Dioscuri and their horses, cannot be traced further back than the time of Constantine, in whose baths they stood, as we are told by Bufalini. The style of sculpture is of the Imperial Age of Rome, and the inscriptions ascribing them to Phidias and Praxiteles are erroneous. 8. Many good specimens of columns, ancient ornamental sculpture, and other monuments may be seen in the following places. The Churches of S. Lorenzo and S. Agnese fuori le Mura; S. Maria in Cosmedin, di Aracœli, degli Angeli, and in Trastevere. The Villas Ludovisi, Borghese, Albani, and Spada. [Illustration: GEOLOGICAL MAP OF ROME] INTRODUCTION TO CHAPTER IX. The geological strata found on the site of Rome and in its immediate neighbourhood divide themselves into three principal groups. The oldest of these is a marine formation, and exhibits itself upon the Vatican, Janiculum, and Monte Mario. The second, of which all the hills on the eastern bank and the district of the Campagna are composed, is of volcanic origin, and consists chiefly of beds of tufaceous matter erupted from submarine volcanoes and more or less solidified. The third, which appears in the hollows of the Tiber valley, is a fresh-water formation, and is found on the slope of the hills on both banks of the river. [Sidenote: Marine formation.]