IOWA ACADEMY OF SCIENCES. 151 



sedimentation has gone on without material interruption dur- 

 ing an even protracted orogenic movement, the line for delimit- 

 ing the various formations may not always be clearly discern- 

 ible and might not, with existing data, be recognized; but with 

 the detailed mapping of the country by the various official 

 geological surveys, the materials are either at hand, or soon 

 will be, for sharply defining all the places where the lines 

 of demarkation should be properlj drawn. These lines, when 

 once made out, and when once properly considered, are as far- 

 reaching, and as universal in application, as those of any 

 classificatory system probably ever can be made. Where the 

 sequence of events has been continuous, lines drawn through 

 the very middle of a rock succession are not entirely arbitrary, 

 but in accord with the history more clearly recorded else- 

 where. 



While orogenic movements vary greatly, both in intensity and 

 extent, they are probably as wide reaching in their effects 

 as any one regional force can be that is of use in geological 

 chronology. They may be rarely or never continental — cer- 

 tainly not world-wide in extent — but the different parts of 

 a given continent may be successively and repeatedly affected so 

 that a given region may be subjected to the influences from 

 several centers of activity. The records of these movements 

 for the continents thus overlap and interlock in such a manner 

 that from all a moderately complete network is evolved, upon 

 which may be arranged, in proper chronological position, 

 the minor episodes. With the comparison of different continents, 

 the difficulties are greater, but there are some lines which 

 surely can be found that are common to both, just as in the 

 case of the various provinces of a single continent. 



In coming down to the lesser stratigraphical groups, as the 

 series, the stages and their subdivisions, the various sub- 

 ordinate or local criteria of correlation may be applied in 

 defining the several members. The leading considerations are 

 the geographical distribution, the lithological characters, the 

 stratigraphical delimitation, and biological definition. In 

 dwelling upon the main characters of each stratigraphical 

 unit, all the physical history must be incorporated. 



In proposing the term orotaxis, denotive of the essential 

 feature in the scheme of geological classification and chronology 

 above outlined, it is not with the idea of advancing an hypothe- 

 sis that is entirely new, but rather of formulating into a 



