462 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



as many times as the number of the diastrophically distinguished "ages" 

 into which the geological time scale is divided. 



This revised conception, however revolutionary it may seem at first, is 

 after all merely a logical refinement of the old, an application of the 

 same ineradicable principles to the minor as well as the major divisions 

 of the geological column. 



Evidence of sea withdrawal has been discovered in many parts of 

 the stratigraphic sequence where none had previously been suspected. 

 This is not because geologists then were either less diligent in their field 

 work or less keen observers than are those of the present generation, but 

 because they had bound their minds and perceptions by certain beliefs, 

 and because they knew and employed only those criteria that under these 

 conceptions alone seemed competent. They recognized shallowing of 

 the seas and shores by the presence of ripple-marked and sun-cracked 

 beds, by trails of creeping things, by tracks, impressions of rain drops, 

 conglomerates, and other phenomena commonly seen along the present 

 shores of the sea; and they knew that tilted marine deposits, the edges 

 of which had been planed down and subsequently covered with approxi- 

 mately horizontal beds, signified previous elevation and subaerial erosion. 

 Presently unconformities between horizontal beds were recognized, and, 

 at first tentatively, then more confidently, interpreted as general elevation 

 into land and resubmergence without conspicuous folding. And so it 

 has gone on and on, perhaps unconsciously, to the present time, when 

 some geologists at least, postulate land areas at almost all such times 

 and places as are indicated by imperfections in the sequence of marine 

 sedimentation. Conviction in this matter of frequent alternation of 

 emerged and submerged conditions seems, indeed, to have progressed 

 so far that the burden of proof now lies with those who deny the propo- 

 sition rather than with those who affirm it. 



PHYSICAL CRITERIA OF STRATIGRAPHIC UNCONFORMITY 



Of the various physical phenomena indicating emergence, stratigraphic 

 unconformities doubtless are tlie most common and most relial)le. But 

 it is only when the coi:itact line is boldly irregular, or when the discord- 

 ance in bedding is great enough to be easily measured, that the uncon- 

 formity is immediately distinguishable from the average bedding plane. 

 These strongly marked cases are. relatively few as compared to the 

 many that are far from conspicuous and which often require corrobo- 

 rative evidence before their true nature can be established. The charac- 

 teristics of clearly defined unconformities are so well known that further 



