The thesis is advanced that virtually all stratigraphic columns exposed on land and penetrated in wells, including those of continental-shelf areas, contain hiatuses of varied nature, the aggregate geologic-time value of which may exceed that of the rock or unconsolidated sediment in the columns. Obscurity of the hiatuses--difficulty in recognizing them--is related mainly to objective features but certainly does not exclude subjective factors. Importance is judged by the amount of continuous time represented by the gaps and by their value in indicating causes for the hiatuses as well as their significance for determining basic principles that apply to time-stratigraphic classification.

End_Page 627------------------------------

Briefly analyzed examples call attention to physical and biological aspects of obscure and even problematical breaks in selected areas at or near boundaries of (1) the Permian and Triassic, (2) the Cretaceous and Paleocene, (3) the Silurian and Devonian, (4) the Devonian and Mississippian, and (5) the Pennsylvanian and Permian; and of minor stratigraphic divisions on (6) the flanks of the Nashville dome and (7) on the northern Mid-Continent stable platform. It is concluded that the obscurity of hiatuses is unrelated to their importance and that pulsatory, more or less localized, differential crustal subsidence furnishes the main control for sedimentary accumulations and the "breaks" within them.

End_of_Article - Last_Page 628------------