Current Action in the Ordocician. — Ruedemann. 385 
somewhat moditied, in the center of the basin where the Gal- 
ena limestone represents the Utica epoch. The change to 
Utica shale and Galena limestone was not a sudden but a 
gradual one, by a series of passage beds, and the Utica shale 
has been shown to extend farther away from the old coast- 
line in a south-westerly direction than towards the northwest, 
in which latter direction it diminishes remarkably in vol- 
ume. 
The direct conclusion, which one might be inclined to draw 
from the foregoing statements, is that the current observed 
in the Mohawk region is responsible for all these phenomena. 
But a comparison of the relatively small area over which the 
influence of the motion of the water has been proved, with the 
immense area involved in that general conclusion, will at once 
demonstrate the great disproportion between the studied area 
and the conclusion, and will prove the necessity of complet- 
ing the work by studying the outcrops of Utica shale in the 
Appalachian region, in the lake Champlain valley — which on 
account of its lying in the direct line of water-motion, observed 
in the Mohawk region, is especially important — in the Cana- 
dian basin, and in the northwest. The writer hopes to have 
an opportunity to do this work next summer. 
Meanwhile it may be allowable to discuss the character of 
the oceanic motion observed in the Mohawk valley under the 
supposition that it was part of a more extensive marine cur- 
rent which brought about the widely extended change from 
the Trenton to the Utica epoch. 
The motion of the water could have consisted in coastal 
tidal currents, in wind-drifts, or it could have been part of the 
general oceanic circulation. 
It is obvious that the receding and advancing tide alone 
could not have produced a motion apparently in only one di- 
rection and parallel to the coast, as the motion south of the 
Adirondacks must be supposed to have been. Powerful tidal 
currents sweep along many shores. They are best known from 
the straits around Great Britain and from Long Island sound, 
but they occur as real currents only in straits, while along 
open shores the translation action of the tide is only small. 
The Ordovician sea. which was at the present site of the Mo- 
hawk valley, was, however, open towards south and therefore 
