Fig. 53.
Cainozoic Ironstone with Leaves (Banksia ? marginata, Cavanilles).
Slightly enlarged. Below Wannon Falls, Redruth, Victoria.
Clay ironstone nodules (sphaerosiderite) have generally been formed as accretions around some decaying organic body. Many clay ironstone nodules, when broken open, reveal a fossil within, such as a coprolitic body, fern frond, fir-cone, shell or fish.
Oolitic ironstones are composed of minute granules which may have originally been calcareous grains, formed by a primitive plant or alga, but since replaced by iron oxide or carbonate.
The Tertiary ironstone of western Victoria is found to contain leaves, which were washed into lakes and swamps ([Fig. 53]); and the ferruginous groundmass may have been originally due to the presence of diatoms, though this yet remains to be proved.