IOWA ACADEMY OP 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 properly 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 
