In "The Seven Storey Mountain," Thomas Merton describes going to Rome as a tourist, but not really seeing the city until the day he walked into the church of Santi Cosma e Damiano, on the edge of the Roman Forum, and stood face to face with the mosaic in the apse. It changed his life. As he started tracking down the other mosaics in the churches in Rome, he says that, without really understanding what was happening, he became a pilgrim.

Finding the mosaics in Rome will take you all over the city. Santa Costanza and Sant'Agnese fuori le Mura are in the northeast, Santa Cecilia and Santa Maria in Trastevere are in the southwest, and the rest of the churches make a large circle around the Colosseum. You can see all of them in a few days. And, though the journey may not change your life as it did Merton's, it will certainly enhance your visit to Rome.

In a city full of Renaissance and Baroque wonders, the mosaics, made of tiny pieces of colored glass set into mortar, let you explore far earlier times--the Rome of the 4th through 13th centuries. The merging of classical Roman art with Byzantine tradition gives the mosaics their particular magic. From classical Roman art they take a love of earth, nature, drama and scenes from everyday life; and from Byzantine art an abstract elegance, a reaching out to eternity.

The place to begin is Santa Costanza (on via Nomentana, the long, straight street that begins at Piazza del Quirinale as via del Quirinale turns into via XX Settembre, and finally becomes via Nomentana). Constantia, the elder daughter of Emperor Constantine, built this mid-4th-century mausoleum for herself and her sister. It is the earliest building with a major mosaic ceiling anywhere. Ancient Romans built mosaic floors; the Christians built mosaic ceilings as well as ornate inlaid floors. The spirit in this ceiling is Roman, but there is a lightness, a touch of humor, that makes it unique. The mosaic ceiling moves from geometric patterns over the entrance to lush designs of birds and branches in Paradise. In between are winged putti, all kinds of fruit, and men with oxcarts harvesting grapes--a hedonist's delight. A happier burial chamber is hard to imagine.

Next door to Santa Costanza, in the same compound, you can witness the other side of Roman mosaic art, the Byzantine. Sant'Agnese fuori le Mura (St. Agnes Outside the Walls) has a 7th-century apse of elongated, solitary figures against a gold background, an early example of Byzantine serenity. There is a sweetness about St. Agnes as flames lap at her feet that makes her seem, despite her regal garb and stance, like a friend next door. That kind of charm animates almost all of Rome's mosaic art.

Three of Merton's favorite churches, slightly earlier than Sant'Agnese, are definitely Roman in spirit: Santa Pudenziana, Santa Maria Maggiore and Santi Cosma e Damiano. The 4th-century apse mosaic in Santa Pudenziana (near the main train station) is so Roman that it catches you off guard. The disciples look like senators debating in the Forum. Jesus, presiding, lounges on his throne.

The 5th-century mosaics along the nave in Santa Maria Maggiore (up the hill from Santa Pudenziana) present the stories of Abraham, Sarah, Isaac, Jacob, Moses and Joshua, with bold gestures and wide-eyed faces. They exemplify the best of Roman storytelling. There is much emotion in those simple designs: the anxiety in the parting of Abraham and Lot, Abraham's humility as he meets the three angels, Sarah's incredulity as she prepares food for the angels, the terror of the men who are carrying the ark of the covenant, Moses's wariness as he commands the waters of the Red Sea while worrying about the Egyptians on the other side. (Here you will need a good pair of binoculars to appreciate the mosaics fully.)

The last of the great early mosaics is the 6th-century apse in Santi Cosma e Damiano (on the northern edge of the Roman Forum). The power of this work comes as a surprise. Seeing it, you understand why it had such an effect on Thomas Merton. No other mosaic in Rome is so assertive. Christ in a gold robe stands on red, blue, green and yellow clouds against a deep blue sky. He gestures toward us. Every figure in the composition is related to all the others. No one stands aloof. There is in this mosaic a sense of physical presence, of motion and great individuality. All the power of the Roman classical tradition is here in Byzantine form.

From the 7th to the 9th century, the art of the mosaic was quiescent. Then, in 817, Paschal was elected pope. Iconoclasm was in full force in the Byzantine world of Constantinople, where from 726 to 842 the veneration of images of Christ, Mary or the saints was forbidden. Rome became a refuge for artists from the east who still wanted to portray the human figure. Paschal put them to work, giving us in his seven years as pope three of the loveliest mosaics in Rome: Santa Maria in Domnica, the chapel of San Zenone in Santa Prassede and Santa Cecilia.

Santa Maria in Domnica (a short hike south of the Colosseum) was Paschal's church before he became pope. It has my favorite mosaic. The sweet naivete in the face of Mary with her Mona Lisa smile, along with the horde of angels around her, make this mosaic irresistible, as does Pope Paschal himself--who, with his unruly hair and the square halo of the living, kneels before Mary, fondling her foot.

The jewel among the mosaics of Rome is the chapel of San Zenone in the church of Santa Prassede (close to Santa Maria Maggiore). The chapel, to the left of the entrance, is a mausoleum that Pope Paschal built for his mother, Theodora. It is easy to find her in the mosaic. She has a square blue halo, which indicates that her son built the chapel for her while she was living. The mosaics surround you in that tiny chapel. The design is rich; the colors brilliant. And outside the chapel in the main apse, there is Paschal proudly holding the church, his offering, in his arms. Few other places in Rome are so immediately satisfying.

The third of Paschal's churches is Santa Cecilia in Trastevere, another 9th-century combination of Byzantine form and Roman charm. Red, blue and green clouds surround Christ who, with large quizzical eyes, blesses the faithful with a Byzantine blessing, his finger forming a circle with his thumb. But the winning touch in this mosaic is the way Santa Cecilia, patron saint of musicians, has her arm around Pope Paschal, her hand resting on his shoulder.

Three centuries after Pope Paschal, mosaic art blossomed again in Rome. A short walk from Santa Cecilia takes you to Santa Maria in Trastevere, with its beautiful 12th- and 13th-century mosaics--much more Byzantine in the emotional distance they project in those faces focused on eternity. Look for the gentle face of the mystical lamb in the middle of the row of sheep; the shepherd entertaining his sheep with a recorder, to the apparent dismay of a nearby ram; and the figures holding a sheet full of earth's bounty on either side of the arch (on the right the sheet is so full they have to sit down).

But the delight of 12th-century Rome is San Clemente, back across the Tiber from Trastevere. There, in brilliant bright colors on a gold background, a crucifixion with Mary and John standing beside the cross is surrounded by plants and animals and scenes from everyday life. There are birds feeding their young, a woman feeding chickens, an owl, a bird in a cage, deer drinking from the river Jordan, peacocks, a snail, a goat herd, a lamb about to lick a shepherd's hand, a bird capturing a lizard, a naked figure riding a dolphin, plus a couple of demons. There are even people in the doorways of Bethlehem and a rooster on the steps in Jerusalem waiting to startle Peter. You could spend hours looking through your binoculars.

Rome still has one more mosaic treat, a late one, from a 19th-century medieval revival by artists who wanted to go back to the art before Raphael, before the Renaissance. The Anglican-Episcopal church of St. Paul's Within the Walls (on via Nazionale at via Napoli, just down from Stazione Termini) has an apse mosaic by the great pre-Raphaelite Englishman Edward Burne-Jones. It was his largest project, and he did it without leaving England.

The lilting rhythmic row of angels in the clouds (completed after Burne-Jones's death) is as lovely as anything you will see in Rome. The long, tapered angel of the annunciation, the somewhat petulant Archangel Michael, and the very pre-Raphaelite Adam and Eve adoring a beefy Christ on the Tree of Life are enchanting.

The 19th century did its part. For the time being, the mosaic pilgrimage in Rome ends here.

Edward Mark lives in Cambridge, Massachusetts.

DETAILS: Mosaics in Rome

ANCIENT ROMAN MOSAICS: To see the mosaics Romans were doing before they came into contact with Byzantine art, go to the reopened Galleria Borghese. The floor mosaics, made not of glass but out of pebbles and pieces of marble, show the Roman mosaicists' delight in individual faces and in nature. Some think the Roman interest in birds and plants is an imitation of Persian mosaics. The Borghese, with its great paintings by Raphael and Caravaggio, is so popular you must call ahead to make a reservation. It's easy. They speak English. From within Rome call 06-32810. You can also see classical Roman mosaics at the newly reopened Palazzo Massimo, which houses the Museo Nazionale Romano (piazza dei Cinquecento near the Stazione), which also has glorious frescoes from the House of Livia.

MINIATURE MOSAICS: The Vatican Museum has a fascinating room of 18th-century micro-mosaics. On your way to the Sistine Chapel just after the long hall of maps, turn left into the Rooms of St. Pius V. It is worth the detour.

GETTING AROUND: The 1998 "Blue Guide Rome" by Alta MacAdam is very good and tells you where all these churches are. The maps in the Michelin green guide to Rome are also useful. The bus map available at the airport train station is essential if you use the Roman bus system, which is excellent. Buy bus tickets or, better yet, a weekly pass at a tobacco shop. Do what the Romans do and watch out for pickpockets on the buses.

Churches tend to be open from 9 a.m. to noon, and from 4 to 6 p.m., but not always on Sunday and Monday afternoons. The tourist offices have a church map with times.

WHERE TO EAT: Ristorante Leonetti (via Farini 56-60), just a block away from Santa Maria Maggiore, serves excellent food at reasonable prices ($25 per person with house wine). Gioia Mia (via degli Avignonesi 34), near the Piazza Barberini, is a down-home place where any of the specialties of the house are inexpensive and delicious ($20 per person).

RECOMMENDED READING--AND A TIP: Walter Oakeshott's "The Mosaics of Rome," now out of print, is worth finding in a library. And carry a good pair of small binoculars, such as the Nikon Travelite III, when you visit the mosaics.

--Edward Mark

CAPTION: A sampling of Roman church mosaics: Clockwise from top, the ornate art of Santa Maria in Domnica, Santa Costanza and Sant'Agnese fuori le Mura.