384 The American Geologist, June, i897 
and altered us it approaches the Oliio valley, and is finalU' 
lost, it becomes also apparent that the cotemporary Galena 
limestone has been found as far northeast as on the Escanaba 
river in the peninsula of Michigan (Hall), but going northward 
also, it becomes reduced and interstratified with shales. Cham- 
berlin (Geology of Wisconsin, vol. ii) reports a similar change 
in the Galena limestone towards the northwestern portion of 
Wisconsin. The modification consists mainly in the introduc- 
tion of more clayey material in the form of shaly leaves and 
l)artings. The change is gradual and progressive from forty 
to fifty miles. 
The statement that in Iowa, Wisconsin and Minnesota tiie 
Trenton limestone has been found to pass into the Galena by 
slow stages proves that, exactly as in the east, the change was 
not a sudden but an oscillatory one. 
From the distribution of the Galena limestone in the nortii- 
west, and especially from its occurrence on the Escanaba riv- 
er, it seems that this limestone facies of the Utica shale ap- 
proaches the coast in the northwest much more than it does 
in the east. This observation, together with the reported 
thinning out* and final disappearance of the Utica shale to- 
wards the northwest along the Archoean area, and with the re- 
markably wide extension of the Utica shale through Ohio and 
to Kentucky — where the lower part of the Cincinnati shale by 
the presence of Utica graptolites and Triarthriis hecki is still 
characterized as a deposit of the Utica epoch — are of signifi- 
cance in regard to the nature of the change, for they indicate 
a farther transference of the mud in the direction of the mo- 
tion of the water in the Mohawk region, than towards the 
northwest. 
It has so far been demonstrated that in the region south of 
the Adirondacks, the close of the Trenton and the beginning 
of the Utica epochs evidently stood in causal connection with 
the coming in of a sediment-bringing west southwest current, 
and not with changes in the sea level. It has further been 
shown that the change from the Trenton to the Utica forma- 
tion took place throughout the north and east of the Appala- 
chian basin where the shale overlies the limestone, and also 
*On Manitoulin island the Utica shale has a thicknes of only 50 
feet. 
