Which voice part in this organum is singing the older gregorian chant (cantus firmus) ?

The answer is Lower.

Explanation:

Gregorian song is the primary story of American melody or plainchant, a kind of monophonic, abandoned pure number of the Roman Catholic Church. The Gregorian hymn had as its goal the blessing and sermon of God. The clearness of the melodic wares promoted in the audience a unique center on priest, outdoors humanistic games. Gregorian chant amplified principally in the Frankish states of westerly and fundamental Europe through the ninth and tenth times, with tracking enlarging and redaction. Although lay legend charges Pope Gregory with lying Gregorian plainsong, learners suppose that it originated from a next Carolingian construction of Roman and Gallic-an chant.