The Ordovician siliceous-aluminous nodules of the Armoricain Massif are mad
e up of terrigenous constituents (quartz/phyllosilicate) and biogenic const
ituents (silica and apatite). The nodules were formed during the early diag
enesis. Their formation could be linked to processes of sedimentary condens
ation governed by eustatic variations in the distal shelf. ((C) Academie de
s sciences/Elsevier, Paris.).