ABSTRACT
Radiolarians are not found in all ancient oceanic deposits. For a long time they were mosdy recognised in friable siliceous deposits such as diatomites or hard siliceous rocks named radiolarites or cherts. Today they are observed in almost all kinds of sedimen tary rocks, both siliceous (radiolarites, bedded cherts) and carbonaceous (limestones, marls and shales). Their abundance in siliceous and carbonaceous formations results from an intricate diagenetic process that transforms both the radiolarian assemblages and the embedding oceanic sediments.
