ALASKA GEOLOGY 
13° 
lower Jurassic (Lias) or to the Eocene. Considering the 
evidence from the side of the genera represented at 
Kadiak, and their geologic distribution in Europe, it points 
perhaps quite as strongly to the Eocene as to the Lias. 
Thus Helminthoida occurs there only in the Eocene, but 
Helminthopsis belongs to the upper Lias ; Palaeodictyon 
is more characteristic of the Eocene Flysch than of the 
Jura, but the reverse is true of Cancellophycus , while 
Chondrites is about equally common in the upper Lias 
and the Flysch. Taking this generic evidence alone into 
account, the question of age could not be determined ; 
but when we extend the comparison to specific alliances, 
and take into account the fact, already noted, that I I el- 
minthoida occurs in America as low as the Lower Car¬ 
boniferous, the case clears up very materially. 
None of the Kadiak species of HeIminthoida are spe¬ 
cifically identical with any of the described European 
species. On the other hand, II. suhcrassa , II abnormis y 
and H. vaga compare quite as closely with the unpub¬ 
lished Lower Carboniferous species and with the figure 
of the Silurian Crossoftodia scotica published by Nichol¬ 
son and Ethridge in their Monograph of the Silurian Fos¬ 
sils of the Girvan District in Ayrshire, as with the Eocene 
H crass a Schafhautl. 
We describe two species of Palceodictyon , and both are 
identified with Swiss Eocene forms figured by Heer, 
though the larger one of the two Kadiak species has 
seemed to us to require separation as a variety. In these 
two fucoids we have the only specific evidence upon 
which the age of the Yakutat slate might be determined 
as Eocene. But carefully analyzed it turns out that even 
here the evidence is scarcely satisfactory, and certainly 
not conclusive. In the first place both P1 magnum and 
P. singulare occur also in rocks that have been, probably 
erroneously, referred to the Lias. Next, at least one of 
