These are overlaid by conglomerates, limestones and clay slates of very doubtful age, the most interesting being a radiolarian clay which occurs on the south side of the Matinang Mountains, at the north end of Lake Posso, &c.; it may correspond with the radiolarian cherts of Borneo.
Red and grey cherts, which have not so far yielded undoubted organic remains, occur in this series, and it has in consequence been compared with the Arenig rocks of southern Scotland.
Some cherts contain tests of radiolaria, and correspond fairly closely to the siliceous radiolarian oozes which are gathering at the present day at the bottom of some of the deepest parts of the oceans.
Brownish cherts are found in the English Greensand; these often contain remains of sponges.
In the Kapoewas district radiolarian cherts supposed to be of Jurassic age are met with.