(F.C. Coll.)
FOSSILIFEROUS ROCKS.
Section I.—ARGILLACEOUS ROCKS.
Under this head are placed the muds, clays, mudstones, shales and slates. MUDS are usually of a silty nature, that is, containing a variable proportion of sand (quartz) grains. Such are the estuarine muds of Pleistocene and Recent age, containing brackish water foraminifera and ostracoda, and those shells of the mollusca usually found associated with brackish conditions. Lacustrine mud can be distinguished by the included freshwater shells, as Limnaea, Coxiella (brackish), Cyclas and Bulinus, as well as the freshwater ostracoda or cyprids ([Fig. 40]).
CLAYS are tenacious mud deposits, having the general composition of a hydrous silicate of alumina with some iron. When a clay deposit tends to split into leaves or laminae, either through moderate pressure or by the included fossil remains occupying distinct planes in the rock, they are called SHALES.
Clays and Shales of marine origin are often crowded with the remains of mollusca. The shells are sometimes associated with leaves and other vegetable remains, if forming part of an alternating series of freshwater and marine conditions. An example of this type of sediments is seen in the Mornington beds of the Balcombian series in Victoria.
MUDSTONE is a term applied to a hardened clay deposit derived from the alteration of an impure limestone, and is more often found in the older series of rocks. Mudstones are frequently crowded with fossils, but owing to chemical changes within the rock, the calcareous organisms are as a rule represented by casts and moulds. At times these so faithfully represent the surface and cavities of the organism that they are almost equivalent to a well preserved fossil ([Fig. 41]).
SLATE.—When shale is subjected to great pressure, a plane of regular splitting called cleavage is induced, which is rarely parallel to the bedding plane or surface spread out on the original sea-floor: the cleavage more often taking place at an appreciable angle to the bedding plane. The graptolitic rocks of Victoria are either shales or slates, according to the absence or development of this cleavage structure in the rock.