130 ALASKA GEOLOGY 



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 ; Palceodictyon 

 is more characteristic of the Eocene Flysch than of the 

 Jura, but the reverse is true of Cancellopkycus, 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 Hel- 

 minthoida occurs in America as low as the Lower Car- 

 boniferous, the case clears up very materially. 



None of the Kadiak species of Helminthoida are spe- 

 cifically identical with any of the described European 

 species. On the other hand, H. subcrassa, H. abnormis, 

 and H. vaga compare quite as closely with the unpub- 

 lished Lower Carboniferous species and with the figure 

 of the Silurian Crossopodia 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. crassa Schaf hautl. 



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 P. magnum and 

 P. singulare occur also in rocks that have been, probably 

 erroneously, referred to the Lias. Next, at least one of 



