128 ALASKA GEOLOGY 



simulating some of the later fucoids, though insufficiently 

 studied to justify a decision concerning their nature, are 

 perhaps chiefly referable to trails of Crustacea and to inor- 

 ganic causes, the American Paleozoic rocks contain but 

 three or four notable horizons for fucoids. The first of 

 these comprises the Frankfort and Lorraine divisions of 

 the Cincinnati series, the second the Medina and Clinton 

 rocks, constituting the basal members of the Silurian, the 

 third the shaly sandstones of the Boston Group of the 

 Lower Carboniferous of northern Arkansas. Fucoids 

 occur in most of the other Paleozoic formations, but in 

 the three horizons mentioned they are more abundant and 

 much more varied in character than in any of the other 

 divisions. Of these other divisions the Erie and Waverly 

 shale in America and the probably nearly corresponding 

 beds in Europe from which Ludwig and others have de- 

 scibed many forms, may be mentioned as ranking next 

 among Paleozoic formations to the Cincinnati, Medina, 

 and Boston rocks in affording fucoids in abundance and 

 variety. 



The strata of the Coal Measures, Permian and Trias ap- 

 parently everywhere are strikingly poor in fucoids, but 

 when we reach the Lias we meet with a wealth of forms 

 in Europe rivaling, if not exceeding, the representation in 

 any Paleozoic horizon. In the upper Jura and middle Cre- 

 taceous they are again inconspicuous, but in the central 

 European deposits of Eocene age, especially the Flysch of 

 Switzerland, they occur once more in great abundance. 



One of the most striking features of these successive 

 marine floras is the extraordinary uniformity of expression 

 running through them all. This is particularly noticeable 

 when we compare the Ordovician types with those found 

 in the Lias and, in a less degree, with those in the Eocene 

 Flysch. It is true each of these horizons is distinguished 

 by its peculiar forms, but the others, among them the 



