Photomicrographs of skeletal grains(I)

This photograph shows a limestone with abundant molluscan casts. In this case shell moulds have been infilled with a few large calcite crystals. Gastropods can be seen, both in long section(lower right)and transverse section(lower left). The long straight shells are bivalve fragments. Careful inspection shows that the long valves in the upper left have a two-layer structure-a thick layer of coarse sparite and a thin layer with a different structure. The rock matrix is micritic sediment.

This photograph illustrates a section through a large thick-shelled gastropod, again preserved as a cast. The outer margin of the shell is picked out by a thin calcite layer, not more than 0.5mm thick at this magnificaiton, but the inner margin is only clear where sediment has partially filled the internal cavity. The sediment around the shell contains abundant small peloids.

This photograph shows two large pink-stained oyster fragments, each having a foliated internal structure. Fragments of oysters may be difficult to distinguish from brachiopods, although their thick shells with a rather irregular foliated structure are characteristic. Note also how the left-hand end of the upper fragment is upturned and splitting. The rest of the sediment comprises broken-up bioclasts set in a blue-stained ferroan calcite cement. The white areas are holes in the section.

This photograph shows a broken brachiopod of which parts of both valves are present and surrounded by a micrite envelope. The fibrous structure is clearly visible, as are fine tubes at right angles to the shell wall, filled with blue-stained ferroan calcite cement. These are 'endopunctae' and they characterize some proups of brachiopods. The sample also shows a good example of coarse, blue-stained ferroan calcite cement.

This photograph shows a number of small impunctate brachiopods with the large pedicle valve and smaller brachial valve complete. The roughly elliptical fragment in the lower center is a section transverse to the length of the fibres making up the shell wall, and shows a characteristic fine net-like structure.

This photograph shows transverse sections through severral spines. They have a structure similar to the brachiopod valve with a foliated inner layer and an occasionally preserved outer prismatic layer. The section through the large spine in the upper left of the picture shows part of the prismatic layer preserved. Note how the shape of the spine gives the foliated layer a concentric structure.

This photograph shows a crinoidal limestone in which the sediment is 75% crinoids. Note the speckled appearance of the plates, most of which have uniform interference colors and are thus single crystals, although the ossicle in the upper left comprises two crystals, one showing a greenish color and one a red color under crossed polars. The clear spar surrounding some of the crinoid fragments is a cement.

This photograph shows one complete transverse section of a spine(lower right of the field), togither with a smaller broken fragment. Echinoid spines are circular or elliptical in cross-section and show a variety of radial structures. Like other echinoderm fragments, they are single crystals.