Closely allied to this marble is an ancient species which puzzles most visitors by its Protean appearance. Its tints are always neutral, but they vary in depth from the lightest to the darkest shade, and are never mixed but in juxtaposition. Dirty yellows, cloudy reds, dim blues and purples, occur in the ground or in the round or waved blotches or crooked veins. It has a fine grain and a dull fracture. This variety of Africano is known by the familiar name of _Porta Santa_, from the circumstance that the jambs and lintel of the first Porta Santa--a Holy Door annexed by Boniface VIII. to St. Peter's in the year 1300--were constructed of this marble. The Porta Santa, it may be mentioned, was instituted in connection with a centenary jubilee, but afterwards the period of formally opening it was reduced to fifty years, and now it is shortened to twenty-five. On the occasion of the jubilee, on Christmas Eve, the Pope knocks three times with a silver hammer against the masonry with which it is filled up, which is then demolished, and the Holy Door remains open for a whole twelvemonth, and on the Christmas Eve of the succeeding year is closed up in the same manner as before. A similar solemnity is performed by proxy at the Lateran, the Liberian, and the Pauline Basilicas. In all these great churches, as in St. Peter's, the jambs and Lintel of the Holy Door are of Porta Santa marble. This beautiful material was brought from the mountains in the neighbourhood of Jassus--a celebrated fishing town of Caria, situated on a small island close to the north coast of the Jassian Bay. From this circumstance it was called by the ancient Romans _Marmor Jassense_. Near the quarries was a sanctuary of Hestia, with a statue of the goddess, which, though unprotected in the open air, was believed never to be touched by rain. The marble, the most highly-prized variety of which was of a blood-red and livid white colour, was used in Greece chiefly for internal decoration. It was introduced in large quantity into Rome, and there are few churches in which the relics of it that existed in older buildings have not been adapted for ornamental purposes. Among the larger and finer masses of Porta Santa may be enumerated two columns and pilasters which belong to the monument of Clement IX., in the Church of Santa Maria Maggiore, and which are remarkable for their exceedingly fine texture and the unusual predominance of white among the other hues; four splendid Corinthian pillars, considered the finest in Rome, in the nave of Sta. Agnese; the pair of half columns which support the pediment of the altar in the Capella della Presentazione in St. Peter's; and the basin of the handsome fountain in front of the Pillar of Marcus Aurelius in the Piazza Colonna, constructed by the architect Giacoma della Porta out of an enormous mass of Porta Santa found lying on the ancient wharf.