Scanning electron microscopy reveals that ooids in this marine unit are of biogenic origin in the sense that endolithic and epilithic algae, fungi?, and mucilage played primary roles in constructing laminae in the cortex of each ooid. Three end-member types of original laminae are recognized (1, 2, 3, below); each is made up of fundamental building blocks of aragonite crystals typically shaped like miniature batons. (1) Filaform (common): laminae composed of a network of calcified algal or possibly fungal filaments, or both. Batons in such laminae are typically less than 1 µ long and most are randomly oriented relative to the nucleus. The batons formed within a mucilaginous sheath surrounding the original algae or fungi. Filaments in such laminae could go through lif as epiliths or begin as epiliths that became endoliths which eventually became epiliths once again. (2) Spheroform (rare): laminae composed mainly of calcified spherical bodies of algal or fungal origin. (3) Sheet (common): pavement-like layers of batons that are typically 0.5 to 2.0 µ long and are oriented tangentially relative to the nucleus. Most batons in type (3) laminae originated in a layer of mucilage or mucous that did not surround algae or fungi, but which enveloped all or much of the surface of a developing ooid. Examples of typical sequences of laminae development are (1)-(1)-(1) etc, (3)-(3)-(3) etc, and (1)-(3)-(1)-(3) etc, possibly with a final type (2) lamina. Contrary to evidence from many modern marine aragonite ooids, filamentous algae was not a constructive fact r in the formation of all laminae in the cortex of all ooids in the Miami Limestone.

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