ORIGIN OF CERTAIN FOLDED SLATES IN ALASKA 13 
severe to distort pebbles in the conglomerate and to develop secondary 
cleavage in the shales. A number of the pebbles and small bowlders 
were dug out of the matrix, and carefully examined for surface mark- 
ings, such as glacial striae, but without success. ‘The shale usually 
adheres more or less closely to the pebbles imbedded in it, as if it 
had been actually welded to their surfaces. On this account no 
favorable exteriors were obtained. 
The age of the formation is not yet definitely known. The Yak- 
utat series has been assigned to the Jurassic by Ulrich, on the basis 
of fossils found in rocks near Kodiak which are thought to be the same 
age; but Wright? is inclined to regard it as somewhat older—probably 
late Carboniferous. 
The interpretation of the shale conglomerate is beset with some 
difficulties arising from insufficient data, but the facts at hand seem, 
nevertheless, to indicate that the deposits are of glacial origin. 
The lack of striations on the bowlders is believed to be largely due to 
subsequent deformation which has defaced or obscured the original 
surfaces on which such markings may have been made. ‘The 
large size and the variations in both size and composition among the 
bowlders seem incompatible with the hypothesis that the beds have 
been formed entirely by aqueous currents. The subangular yet 
irregular shapes of the bodies are also more suggestive of glacial 
origin than of any other. ‘There is one feature of the formation which 
is sufficient, however, to prove that even if glacial it is not an ancient 
deposit of till or moraine, namely, the distinct stratification. ‘The 
shale matrix was evidently accumulated in quiet waters where condi- 
tions favored the settling of clays and silts in successive horizontal 
layers. A suggestive condition now prevails in this very region: in 
the broad estuary called Yakutat Bay, fine sediments are doubtless 
now accumulating; at the same time abundant icebergs drift out 
from the glaciers at the head of the bay and eventually melt before 
they reach the Pacific. Obviously the bowlders and finer débris 
enclosed in this floating ice are strewn over the bottom of the bay, 
and it may be supposed that the result is a stratified argillaceous 
tE. O. Uhlrich, Harriman Alaska Expedition (1904), Vol. IV. 
2C. W. Wright, in conversation with the writer at the close of the field season 
of 1906. 
