Photomicrographs of skeletal grains(II)

This photograph shows a transverse section and parts of two longitudinal sections of the colonial rugose coral Lithostrotion. Note the thick outer wall and septa seen in the transverse section. The columella and thin tabulae are clearly visible in the longitudinal section. Parts of the coral walls have been silicified(brownish color). The pore-filling material is mainly sparite cement with some micritic sediment between the corallites.

This photograph is a transverse section of a stick-like bryozoan colony, showing the overall rounded shape of the 'stem' and of the zooecia within. Some of these have been infilled with fine sediment(supper right of fragment) but most have a blue-stained, ferroan calcite cement infill. In general, most bryozoans had calcite hard parts and a liminated wall structure is preserved.

In this photograph, the two circular, concentrically-laminated grains stained red-brown are brachiopod spines. These are encrusted by a bryozoan. Note the thick calcite wall of the bryozoan and the pores of different sizes within the skeleton, filled with pink-stained non-ferroan calcite cement. Some fragments of fenestrate bryozoans can be seen along the left-hand side of the photograph.

This photograph shows a cross-section of a trilobite(center) and part of a brachiopod shell(base). Note the hooked shape seen at the left-hand end of the trilobite fragment, produced by incurving of the skeleton at its margin. A vein of blue-stained ferroan calcite follows the edge of the skeleton along part of its length. Note that the trilobite is stained mauve and hence consists of slihgtlly ferroan calcite. This contrasts with the brachiopod fragment which is non-ferroan calcite.

This photograph shows discocyclinids, a type of foraminifer with many chambers. The matrix is micrite with many fragmented bioclasts. In general, most of foraminifera are calcite but they show a variety of shapes and wall structures.

This photograph shows a foraminiferal limestone in which the organisms are micrite-walled miliolids. The cement is fine sparite although unfilled pore-spaces remain(e.g. center of field of view). Partly-filled moulds of bivalves can be seen outlined by thin micrite envelopes. These are the elongate curved grains seen on the right-hand side of the photograph.

This photograph shows segments of one of the common forms of codiacean alga, Halimeda, which still occurs today. Living examples contain organic filaments embedded in aragonite. The example shown is from a poorly-consolidated Qauternary sediment which had to be impregnated with resin before a peel could be made. The grey areas between the algal segments and in the holes originally occupied by the filaments are the impregnating medium.

The third group of green algae are the charophytes, although these are sometimes classified separately. They are freshwater plants, occurring in the Mesozoic and Cenozoic, and usually only the reproductive parts(oogonia) are calcified. These are small egg-shaped bodies with various ornaments. This photograph shows three oogonia in cross-section.