TEXTURE - GRAIN OR CRYSTAL SIZES
Size classification should be based on a modified Wentworth scale. Do not try to record size grades without reference to some standard comparator. Shape involves both sphericity and roundness. Sphericity refers to a comparison of the surface area of a sphere of the same volume as the grain. Roundness refers to the sharpness of the edges and corners of a fragment. Angular: Edges and corner sharp; little or no evidence of wear. Subangular: Faces untouched but edges and corners rounded. Subrounded: Edges and corners rounded to smooth curves; areas of original faces reduced. Rounded: Original faces almost completely destroyed. Well rounded: No original faces (flat faces are absent).
It involves shape, roundness, specific gravity and mineral composition as well as size.
CEMENT AND MATRIX
Cement is a chemical precipitate deposited around the grains and in the interstices of a sediment. Matrix consists of small individual grains that fill interstices between the larger grains. Cement is deposited chemically and matrix mechanically. Chemical cement is uncommon in sandstone which is a clay matrix. The most common cementing materials are silica and calcite. Silica cement is common in nearly all quartz sandstones. Dolomite and calcite are deposited as crystals in the interstices and as and as aggregates in the voids. Anhydrite and gypsum cements, are more commonly associated with dolomite and silica. Additional cementing materials include pyrite, generally as small crystals, siderite, hermatite, limonite, zeolites and phosphatic material. Silt acts as a matrix and clay is a common matrix material which may cause loss of porosity either by compaction or by swelling. Argillaceous material can be evenly distributed in siliclastic or carbonate rocks. Coarse grained sandstones have greater permeability than finer ones when the same amount of cementing material is available to both.
FOSSILS AND ACCESSORIES
Microfossils and some small macrofossils, or even fragments of fossils, are used for correlation and may be environment indicators. Any geologists who examine samples should be able to distinguish foraminifera, ostracods, chara, bryozoz, corals, algae, crinoids, brachipods, pelecypods and gastropods so as to record their presence and relative abundance in the sample logger to have available one or more slides or photographs illustrating the principal microfossils. Accessory constiruents may be significant indicators of environment of deposition. The most common accessories are glauconite, pyrite, feldspar, micra, siderite, carbonized plant remains, heavy minerals, chert and sand-sized rock fragments.
POROSITY AND PERMEABILITY
Among the most important observations made in the course of sample examination are those relating to porosity and permeability.