580 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



to another and thus interrupted the process of sedimentation, will per- 

 haps seem to the average geologist the most difficult to grasp. But these 

 together constitute the very backbone of the revision. Without them> 

 the distant correlations largely fall to the ground or remain indefinite 

 and unsystematized as they have always been. 



It is not claimed that the basic ideas are new. The revision merely 

 applies them more generally than was heretofore contemplated. We 

 have long recognized that some of the Paleozoic seas were small, that 

 they occasionally shifted their boundaries and that there must be some^ 

 thing akin to rhythm in the great deformative movements that were- 

 more or less clearly suggested in the stony record. But these phenomena 

 were treated as exceptional and local and not as definitely connected 

 with a general systematically developed diastrophic process. That the 

 strandline moved in obedience to the gradual development of this process 

 and registered its stages, and that it therefore offered a definite and 

 easily applied basis for stratigraphic correlation and classification, was 

 only partially acknowledged. Chamberlin and Salisbury recognized the 

 importance of the factor in so far as the major divisions of tlie strati- 

 graphic column are concerned, but the demonstration of its applicability 

 to minor divisions no less than the major has been left to the present 

 occasion. 



Method of dividing geologic time and definiteness of time U7iit>!. — In 

 stratigraphic taxonomy the first essential is the selection of some method 

 or principle determining when a geologic age has ended and a newer age 

 begun. Obviously such a principle applies equally to all the divisions of 

 the time scale, the larger divisions, up to periods and eras, being but 

 combinations of the minor units. In other words, the base or top, as the- 

 case may be, of a terminal unit of a group, series, system, or era at the- 

 same time correspondingly delimits the division of higher rank of which 

 it forms a part. 



In practice the stratigraphic terms system, series, group, and forma- 

 tion are treated as respectively equivalent to the time terms period, epoch, 

 stage, and age. In fact, however, there is a difference. The latter refer 

 to parts that taken in serial order completely fill geologic time. The 

 former, on the contrary, account for only so much of this time as is rep- 

 resented by sedimentation in places now accessible, l^ew discoveries, 

 however, may at any time necessitate intercalation of distinguishable and 

 hitherto unknown stratigraphic units and corresponding revision of the 

 time units. Moreover, some of the inaccessibly recorded parts are occa- 

 sionally penetrated in deep wells, but with such relatively unimportant 

 exceptions they are truly "lost intervals." 



