A combination of scanning electron microscopy, light microscopy, decalcification, staining with malachite green, and a Lakeside-70 impregnation-decalcification technique shows that biogenic and inferred biogenic structures are present on and within all types of allochems in samples from 12 oolites ranging in age from Holocene to Silurian. The types of structures include borings, mucilaginous filaments, pits, irregular depressions, and globules. The most abundant and best preserved of the structures are in Holocene samples.

Borings are the commonest structure in all types of allochems, regardless of the age of the sample, and appear to be due to blue-green algae and to a lesser extent to fungi. Borings may consist of a single straight or anastomosing structure, or may have a ramiform structure. More commonly, borings form complex reticular, clotted, spongy, or polygonal structures.

In the samples studied, no systematic relation exists between the nature, abundance, and distribution of borings and the development of micrite envelopes in ooids. Most ooids which contain abundant borings do not display any sign of a micrite envelope. Also, borings may be as abundant in ooids as they are in peloids. These new findings support ideas presented previously by Bathurst and by Purdy that either decomposition of organic matter by bacterial activity, or the metabolic activity of the original borer, or both, play a more important role than the borings themselves in the development of micrite envelopes and of recrystallization in ooids.

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