But it is not as a mere amusement, or as a means of collecting pretty souvenirs of travel, that such marble-hunting expeditions are to be recommended.  They may have a much higher value.  The different kinds of marble collected are peculiarly interesting owing to their association with the different epochs of the history of the city and empire; and as the specimens which the geologist obtains throw light upon the formation of the rocky strata of the earth, so the small marble fragments which the student finds in Rome afford a clue to the various stages of its existence.  Indeed, a competent knowledge of the marbles of Rome is indispensable to a clear understanding of the age of its ancient monuments.  An immense amount of controversy has raged round some remarkable building or statue, which would have been prevented had the nature and origin of the marble of which it was composed been first investigated.  The famous statue of the Apollo Belvedere in the Vatican, for instance, was long regarded as an original production either of Pheidias himself or of his school.  But the discovery that the marble of which it is wrought is Lunar or Carrara marble—­which was unknown until the time of Julius Caesar, who first introduced it into Rome—­is of itself a proof that it is not a genuine work of Greek art of the best period, but a monument of the decadence, or a copy of an original, wrought in imperial times for the adornment of a summer palace in Italy.  In numberless other cases, ancient monuments have been identified by the mineral character and history of their marble materials.  The first thing, therefore, which the student during his visit to the city ought to do, is to make himself acquainted with the different varieties of marble that have been found within the walls or in the neighbourhood.  For this purpose the Museum in the Collegio della Sapienza or University of Rome will afford invaluable aid.  In this institution, conveniently arranged in glass cases, are no less than 607 specimens of various marbles and alabasters used by the ancient Romans in the building or decoration of their houses and public monuments.  The collection was made by the late Signor Sanginetti, Professor of Mineralogy in the University, and is quite unique.  A great deal of instruction may also be obtained from the mineralogical study of the thousands of marble columns still standing in the older churches and palaces of Rome, most of which have been derived from the ruins of ancient temples and basilicas.  Several excellent books may also be consulted with advantage—­especially Faustino Corsi’s Treatise on the Stones of Antiquity, Trattato delle Pietre Antiche, which is the most approved Italian work on the subject, and from which much of the information contained in the following pages has been obtained.