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 
