Stratigraphic units are divisions of the rocks of the earth’s crust which in a broad sense parallel the stratification of these rocks. There are many types of stratigraphic units—as many types as there are many criteria on the basis of which rocks may be differentiated stratigraphically. Rocks may be classified stratigraphically on the basis of variations in their visible material constituents—lithology, fossil content, mineral content, etc. Rocks may be classified stratigraphically on the basis of variations in their physical and chemical properties—electrical resistivity, spontaneous electrical potential, radioactivity, chemical constitution, transmission of seismic waves, etc. They may be classified...

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