STRATIGEAPHIC CLASSIFICATION STRUCTURAL CRITERIA 461 



to wash the surface clean of all loose material. That it succeeded in 

 doing so in this case must forever silence the argument against the as- 

 sumption of land conditions without evidence of subaerially produced 

 surface materials. (See figure 17 B^ page 450.) 



PHYSICAL EVIDENCE OF SEA WITHDRAWAL 



It is generally believed by geologists that in past geologic ages marine 

 waters pulsated back and forth over areas now comprised in the conti- 

 nental land masses. The more important of these transgressions were 

 legitimately inferred long ago on good stratigraphic and paleontologic 

 grounds, and they rightly formed the basis on which geological time was 

 subdivided. Progress in our science since its early and middle stages 

 has been largely confined to the accumulation of data and to occasional 

 attempts at refinement in the way of closer discrimination and readjust- 

 ment of the long established major boundaries. More recently the local 

 components of the several systems were subjected to critical study. On 

 comparison it was found that great diversity of expression and develop- 

 ment exists. The fossils were not exactly alike, the lithology was differ- 

 ent, and the stratigraphic boundaries indicated in one area seemed not 

 to harmonize with those in another. Evidently something was wrong. 



The fault lies in holding too long to the idea of great continental seas 

 and in overestimating their durance. Under this primitive and long 

 treasured conception local absence of deposits of certain ages had to be 

 ascribed to removal by erosion; but as the imperfections in the strati- 

 graphic sequence continued to multiply, while the discovery of adequate 

 amounts of erosion material, especially in the interior areas of flat-lying 

 rocks, lagged far behind, and, more important yet, since detailed investi- 

 gations of overlapping formations proved over and over again that their 

 upper beds not only spread farthest, but were often preserved despite 

 long exposure to subaerial conditions, some modification of the prevailing 

 conception became imperative. The belief in broad, deep, and long en- 

 during continental seas — seas that began early in the Cambrian and con- 

 tinued spreading wider and wider until well toward the close of the 

 Ordovician — must be abandoned. In its place we should conceive of 

 smaller, very shallow, and frequently shifting bodies of water, of seas 

 that filled a given basin in one age and were withdrawn in the next, that 

 returned again and again in familiar patterns, though perchance from 

 different quarters, in succeeding geological ages. In short, seas that 

 migrated in and out of the structural basins — sometimes extending far 

 across the continents and at other times limited to much smaller areas — 



