534 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



beds seemed conformable and were of the same general lithologic type. 

 With the aid of fossils, however, not only important breaks in deposition 

 came to light, but also constant and easily recognizable differences in 

 petrological characters were found that by themselves enabled the field 

 geologist to distinguish the several parts of the previously supposed unit. 

 Moreover, the fossils rarely failed to substantiate the persistence of the 

 more striking lithological boundaries. 



Willis's evident pessimism respecting correlations seems grounded in 

 the belief that the continents, though permanent and essentially stable, 

 were locally affected by periodic but otherwise unrelated movements, 

 and that the continental seas were long enduring, often broad and deep 

 interoceanic waterways, the sediments and faunas of which varied greatly 

 from place to place according to nearness or remoteness from shores and 

 ko varying local efficiency of currents. My conception is very different. 

 I see mostly small, shallow, often disconnected basins, occupied at times 

 by Atlantic, and at other times by Gulf of Mexico or by Arctic waters. 

 These were filled and emptied many times, and on each occasion the size 

 and form of the basins differed more or less. As to the continents, I, too, 

 believe they were permanent in general features and that they w^ere 

 affected by many deformative movements, but these movements were 

 always related to a definite, rhythmically progressive plan. Under 

 Willis's view detailed correlations are as impossible, as he believes, 

 because it affords no means of determining the time relations of rela- 

 tively local physical and organic phenomena to the general scheme of 

 geologic events; under mine definite and often very detailed correlations 

 can usually be made because it is based primarily on the displacements 

 of the strandline which, of all geologic processes, were the most wide- 

 spread and most nearly simultaneous in their operation. Under his 

 view, again, the local imperfections of the marine stratigraphic column 

 were largely occasioned by fortuitous conditions of uncertain character 

 and relations and not, as under mine, by the periodic retreat and conse- 

 quent absence of marine waters. Finally, in that it insists on land bar- 

 riers between continental seas and on the integrity of the invasions from 

 the several oceanic basins, my view, in contradistinction to Mr. Willis', 

 offers sound and efficient reasons for the regional variations of the fauna! 

 aggregates. 



Summary of diastrophic criteria. — Concisely stated, the criteria of 

 diastrophism embrace all physical, and to a certain extent all organic, 

 phenomena implying horizontal and vertical movements of the crust of 

 the lithosphere. These deformative movements were not always in opera- 



