Everywhere there are marble columns, sculptures of sacred personages, and 4000 square meters of mosaics all unified according to a complex iconographical plan.
 

Come every day for the rest of your life with a small portable helicopter and you might begin to feel that you have seen everything that this cave of wonders has to offer. Failing that, make more than one visit - preferably at different times of day, to appreciate the varying effects of  light on the mosaics. The lambent interior exudes splendour and mystery, even when bursting with tourists. It is in many ways an exercise in obsession: for six centuries the Venetians continued to add to its treasures, leaving not an inch uncovered.

 . . . The surfaces - all the surfaces - are covered by more than 4,000 square metres of mosaics, the result of six centuries of labour. The finest pieces, dating from the twelfth and thirteenth centuries, are the work of Venetian crafts-people influenced by Byzantine art but developing their own independent style. The chapels and Baptistry were decorated in the fourteenth and fifteenth centuries; a century later, replacements of earlier mosaics were made using cartoons by such artists as Titian and Tintoretto. However, most of these later mosaics are fundamentally flawed by the attempt to achieve the three-dimensional effects of Renaissance painting.

 . . .Notice too the magnificent marble, porphyry and glass mosaics of the floor. These date from the twelfth century, though they have been much restored. In the last century the British architect GE Street managed to block a large-scale programme of replacement by convincing the authorities that the undulating surface of the floor was a deliberate attempt to imitate the waves of the sea. He was too late to save the left aisle, which is noticeably smoother. 
(timeout.com)

Notes:

• Overview of Venice
• Piazza San Marco and Piazzetta
• Basilica Exterior
• See Interior of Basilica and location of Sargent's painting