Current Action in the Ordovician. — Ruedemann. 383 
The general occurrence of the passage beds gives evidence 
that the change from the physical conditions of the Trenton 
epoch to those of the Utica epoch was not a sudden one. but 
consisted in a ti-ansition by numerous oscillations. But after 
the conditions favorable to the deposition of the Utica shale 
had once gained prevalence, they must have continued for a 
longtime. This can be inferred from the extensive geograph- 
ical distribution of the Utica slate and its retaining its char- 
acteristic black carbonaceous slaty character, which shows 
the rock to be a slow deposit of deeper water, and from the 
comparatively large volume, notwithstanding its probably 
slow deposition. 
Walcott further develops the important fact that, while 
all along the coast of the old continent the Utica shale was 
formed, the formation of limestone continued towards the cen- 
ter of the Appalachian basin.* That author shows that the 
same change which terminated the Trenton formation in the 
east and north of the Appalachian basin, and inaugurated the 
Utica epoch, also wrought some change in the fauna of the 
limestone-forming sea in the center; a change which is recog- 
nized in the difference between the faunas of the Trenton and 
the superjacent Galena limestones. While the Utica shale 
has fifty-four species limited to its boundaries, and thirty-six 
derived from the Trenton, the Galena limestone has only nine- 
teen species peculiar to it, and fifty- six passing up from the 
Trenton formation beneath. "This diversity is undoubtedly 
owing to the greater variation in the character of the sedi- 
ments of the Utica slate as compared with the Galena, when 
the change from the Trenton limestone-forming deposit oc- 
curred." While this diversity, on the one hand, proves that a 
change in the physical conditions in the east and north must 
h^ve changed the fauna there, it demonstrates also, on the 
other hand, that this same phj^sical change made its influence 
felt even in the center where the limestone formation continued. 
While Walcott, on the evidence of Prof. Orton's report on 
the geology of Ohio (1880), shows that the Utica shale is still 
met with in wells drilled in northwestern Ohio, but is reduced 
*This term includes the interior continental basin from the Appala- 
chian system to the Mississijjpi. Later the Galena has been referred to 
the Trenton of New York state by several geologists, viz., N. H. Win- 
chell, E. O. Ulrich and F. W. Sardeson, vol. iii, part ii. Final Renort 
Minn. Survey, 189G. ' 
