Extinction is a common phenomenon as shown by the fact that almost all fossil taxa are extinct. But when diverse organism assemblages disappear at, or near, a single geological horizon, it is a temptation to postulate a catastrophic event. Superficial appearances of mass extinction, however, may be misleading. An indication of decline in diversity may not be sufficient to establish the reality of any revolutionary change. There are many obvious and some obscure sources of error involved in sampling the fossil record. Precise time-correlations are usually lacking and taxon ranges, as recorded, are systematically misleading. The standard method of reporting fossil ranges artificially concentrates last occurrences at stratigraphic boundaries. A similar effect may also result from unrecognized sedimentary hiatuses (paraconformities), which may simulate mass extinction events. Many biological revolutions are indeed real, as shown by clues of environmental perturbations on a world scale. Generally, however, mass extinctions were spread over millions of years and can be considered catastrophic only as a final disappearance in an accelerating downward trend in diversity.

Fossil reefs of Permian age in West Texas and southern New Mexico are remarkably well developed and are ideally exposed. This study is an attempt to interpret the environmental conditions under which some of these interesting structures were formed. Three adjacent geologic provinces are characterized by stratigraphically equivalent rocks and fossils of strongly contrasting fades: (1) the Delaware basin with laminated detrital drab-to-black limestones and quartz sandstones and a pelagic fauna; (2) the basin margin, occupied by light-colored very fossiliferous massive reefs and by banks of detrital limestone and dolomite; and (3) a shelf area covered by comparatively thin-bedded unfossiliferous, light-colored rocks, evaporites, dolomite, and quartz sandstone. Regional analysis leads to the following paleoecologic conclusions: The land around the Permian seaway was very low after earliest Permian time, and the climate was warm and dry. The marine faunas are most similar to contemporaneous faunas at low latitudes in the Eastern Hemisphere (Tethys). This probably is a reflection of circumequatorial conditions. The lithologic, paleontologic and structural characteristics of the Delaware basin suggest deposition in quiet waters which at times were at least 1800 feet deep. On the other hand reefs and banks at the basin rim were formed near the surface where wave attack and occasional collapse resulted in a succession of wedges of detrital limesand and talus seaward and lagoonward from the basin rim. These marginal deposits, unlike those of the basin and shelf, are mainly composed of skeletal material reflecting relatively greater organic productivity here of calcareous deposits. The lagoonal, or shelf, deposits are relatively poor in skeletal material. This and other characteristics of the rocks show that environmental conditions on the shelf were unfavorable for many kinds of organisms. Waters of the shelf area probably were generally deeper than at the rim, but it is unlikely that they exceeded a few tens of feet. The regional relationships suggest a shelfward flow of surface waters over evaporating pans where hypersaline waters were trapped behind the low barrier of slightly elevated banks and reefs. Basin waters of nearly normal salinity evidently were continuously renewed through one or more shallow inlets, probably at the south side of the Delaware basin. The depths of the Delaware basin below inlet threshold were little disturbed by this flow, and because of mild winter temperatures there was only limited seasonal turnover. Hence, the deeper waters were generally stagnant.

Abstract Southeast of Florida, outliers of the continental shelf form submerged limestone plains covering nearly 60,000 square miles. These are the Bahamian platforms which, for the most part, are covered by only a few feet of clear sea water. Conditions of sedimentation here must closely resemble those of the limestone shelf seas of the Paleozoic and Mesozoic. Nearly three miles of carbonate deposits have been laid down in the Bahamas since the early part of the Cretaceous period, indicating that there has been a high sustained rate of deposition. Doubtless this has contributed to the persistent subsidence of the area. This area is one of the most instructive in the world for studies of the origins of marine limestones. Calcium carbonate sediments virtually free from terrigenous materials are being formed in easily accessible sites and under a variety of conditions. Oolite and aragonite ooze are being formed on a scale probably unequaled elsewhere. Abundant sediments are also being formed by accumulation of skeletal remains of calcareous algae and invertebrates. Vigorous, newly established coral reefs dominate the scene in places and provide valuable clues about the origins of these structures. Although the region provides many of the conditions supposed by some to be favorable for the formation of primary dolomite, this rock is being formed here only at considerable depth, and the circumstances suggest only a secondary origin. Because of shallowness and clarity of the waters over the platforms, this area is unexcelled for direct visual examination of the sea floor by shallow diving and for documentation on aerial and submarine photographs. This work was undertaken as an introduction to the marine geology and environments of the Great Bahama Bank, and emphasis has been laid on those relationships and processes which might be of value to the historical geologist intent on the general problems of ecological interpretations of fossils and sedimentary rocks. Particular attention was given to the celebrated Andros reefs and lagoon, and about 1000 square miles of this part of the area was mapped and studied at some length. Regional data are presented with respect to conditions under which the various calcareous sediments are being formed.