350 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



factory, I have no opinion to offer regarding the value in years of the re- 

 spective periods and eras. The criteria on which all such estimates have 

 been based are insufficiently understood and moreover seem too variable 

 to aiford trustworthy data. All I am certain of is that geologic time is 

 very long and ample for the slow evolution of the earth and of the life 

 on it. 



Relatively hopeless imperfections of the sedimentary record. — The 

 more important of the breaks in the stratigraphic record, of impor- 

 tance chiefly because they include the hiatuses most likely to remain 

 unbridged, are the intervals of very great general emergence. I refer 

 especially to the intersystemic intervals when the continents were large 

 and the seas confined to the oceanic basins. Some of these emergent in- 

 tervals were very long, and considering geological time as a whole their 

 average duration seems to have increased with time, as did also the aver- 

 age elevation of the continents above sealevel. Their duration, further, 

 seems to be in proportion to the greatness of the emergence, the longest 

 intervals being those in which the continents were largest. Following the 

 idea expressed on page 342, the intrasystemic breaks in the median parts 

 of the continent also were greater than usual in the periods next following 

 and preceding a long intersystemic interval. The continental seas during 

 such periods were on the whole more limited in extent than in periods 

 separated by shorter intervals. Long, intrasystemic intervals of inacces- 

 sible marine sedimentary record are suggested for the Canadian, Waver- 

 lyan, Tennessean, Pennsylvanian, Jura-Triassic, and Comanchean, whose 

 continental seas were relatively restricted, while shorter intrasystemic 

 intervals are indicated for the Ordovician, Silurian, Devonian, and Cre- 

 taceous periods in which the continent, particularly its central and north- 

 ern interior parts, was widely submerged. 



The marine record of these great intersystemic and intrasystemic in- 

 tervals of emergence, being wholly or at least mainly confined to the 

 oceanic basins and the marginal shelf of the continents, is now quite 

 inaccessible. They are, I fear, really "lost intervals." All we can hope 

 to learn concerning the history of these intervals is the unsatisfactory 

 information to be gathered from a study of the phenomena directly con- 

 nected with the breaks in the stratified column and from the more defi- 

 nite though yet very incomplete organic and physical data preserved in 

 the land deposits. 



Doubtless land deposits were made in all of these emergent intervals, 

 but it is no less clear that the longer they endured the more complete the 

 removal of such deposits by the ever-active agencies of baseleveling. 

 Hence, absence of such deposits when extensive lands are indicated by 



