The modern reef is both a morphologic and an ecologic concept. It is a structure which rises more or less abruptly from the sea floor to within a short distance of the water surface. This position and the topographic isolation of the structure are the primary ecologic controls for organisms living on the reef. Thus, rapid changes in depth influence water circulation and chemistry, the availability of light, and the movement of sediment.

The reef biota includes both sessile frame-building organisms and vagrant animals, zonally arranged in relation to an ecologic gradient determined by the environment. It is, however, the frame-builders which are regarded as definitive of "reef" structure.

Many ancient facies assemblies which have been described as reefs differ from such a model. The major areas of subjective argument lie in the interpretation of the depositional environments of the sediments and of the paleoecologic significance of particular organisms and associations.

There is a general lack of knowledge of the internal features of modern structures, and particularly of the influence of their recent growth history upon internal relations. This also contributes to the lack of precision surrounding reef definition.

An analysis of the criteria available for the recognition of ancient reefs shows that other structures are present in the geologic column which pass unnoticed beneath the blanket title of "reef." It is the aim of this paper to differentiate between these bodies and reefs in the modern sense. Their proper description carries both interpretative and economic benefits.