318 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



been great local variation in erosional processes and results. The only 

 real difference is that in the submergent stages^ to which onr knowledge 

 of the pre-Cenozoic periods is largely confined, the size of the continents, 

 and especially the average relief of the lands, was generally much less 

 than now. Therefore, while degradational agencies in the periodic highly 

 emergent stages probably were active enough and occasionally perhaps 

 comparable in vigor and results to those of the present time, those work- 

 ing on the relatively low lands prevailing in the intervening submergent 

 phases must have been correspondingly inferior in both respects. It 

 will be understood, of course, that 1 am referring only to tlie continental 

 areas, whose record is accessible and more or less studied. Concern- 

 ing the extreme marginal lands, the early record of which is scarcely 

 decipherable, these probably were frequently high and as often reduced 

 by erosion. Speculative contributions to their history will be attempted 

 in following sections (see pages 435 to 477). Despite the easily estab- 

 lished fact that in several important respects present conditions along 

 our sea shores are very different from those prevailing in and adjacent to 

 the continental seas of the past, it is common practice among geologists 

 to insist on a strict accounting of the old according to the familiar fea- 

 tures of the present. We know, for instance, that the character of the 

 near-shore bottom of the Atlantic varies rapidly and greatly from place 

 to place. Here we see a clean sand beach, near by a fine mud bottom, 

 and not far away the shore is strewn with great boulders torn from a 

 massive cliff. Similarly extreme, yet today very ordinary, variation of 

 shore conditions seems to have been very rare, not to say impossible, in 

 the Paleozoic epicontinental seas. Their shores were rarely or never pre- 

 cipitous, and the broad interior lands washed by these seas were often so 

 low that the small clastic matter derived from them exerted a scarcely 

 appreciable effect on the character of the deposits along hundreds, yes 

 thousands, of miles of shoreline. The latter extreme in uniformity of 

 sediment is shown by the Lowville, a thin middle Ordovician limestone 

 formation, covering nearly 500,000 square miles of territory. Naturally, 

 the best examples of geographic persistence of lithologic units are found 

 among the limestone formations, but we have also hlaclc shales and blue 

 shales, and even sandstones, that spread over great areas. 



And yet the latest editions of such excellent text books as Dana's 

 Manual (pages 3-98-399), Geikie's Textbook (pages 655-656), and Kay- 

 ser's Lehrbuch (II, pages 6 and 7) contain statements that can not fail 

 to impress the student with the belief that precisely similar variability 

 of bottom and shore as occurs in the ocean basins of today prevailed also 

 in the continental seas of all geological periods. It is not that these 



