Ovotaxi.s : A Method of Correlation. — Kei/es. ;^0i 
While erogenic movements vary greatly both in intensity'' 
and extent they are probably as wide-reaching in their effects 
iis anyone regional force can be that is of use in geological 
correlation and chronology. They may be rarely or never 
•continental, certainly not world-wide, in extent, but the differ- 
ent parts of each continent maybe repeatedly affected so that 
a given region may be subjected to these influences from sev- 
eral centers of activity. 'I'he visible planes of unconformity 
which record these movements for each major region thus 
overlap and interlock in such a manner that from them all a 
moderately complete network may be constructed which will 
bind together in a compact whole and in proper chronological 
order all the minor parts. With the comparison of different 
i^'ontinents the ditliculties 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, stages and their subdivisions, the various subordinate 
or local criteria of correlation that are now in general use 
may be made serviceable in the defintion of the several mem- 
bers. The leading considerations are the geographical dis- 
tribution, the lithological character, the physiographic ex- 
pression, the stratigraphical delimitation, and the biological 
definition. In dwelling upon the main characteristics of each 
stratigraphical unit the whole physical history must be in- 
<'orporated. 
In general geological classification about the only practical 
attempt to use the orotaxial principle has been in the de- 
n)arkation of the grand divisions or systems, and the events 
are commonly referred to as geological revolutions. Thenear- 
<'St approach to the actual application of the idea in any of 
its phases, has been by Irving* in his work on the pre-Cam- 
brian crystallines of the northwest, in which unconformities 
are given great prominence, by McGeef in his investigations 
of the Coastal plain deposits of the middle Atlantic slope, 
in which similarity of origin, or homogeny, is the governing 
factor, and by Davis;J; and others in their physiographic work, 
*U. S. Geol. Sm-., 7th Ann. Kept., pp. 378, 1888. 
tAm. Jour. Sci., (3), vol. xl, pp. 36-41, 1890. 
JNat. Geog. Mag., vol. i, pp. 180-253, 1889. 
