Complex sentence: Pieces 80 to 93 are 14 different settings of Amen in different modes and pieces 94 to 104 are ten settings of Gloria in different modes and an introit.
Split into simpler sentences: Pieces 80 to 93 are 14 different settings of Amen in different modes. Pieces 94 to 104 are ten settings of Gloria in different modes and an introit.