Page:A Treatise on Geology, volume 1.djvu/159

 CHAP. VI.

Now it is remarkable that many of these are fossils which occur in more than one stratum,—often, as the brachiopoda, in several—so that they are thus found to be truly characteristic of the Silurian system, or of large thicknesses of it. Other shells are most abundant in particular deposits—as Serpulites longissimus in Upper Ludlow, Pentamerus Knightii in Aymestry rock, Graptolithus Ludensis in Lower Ludlow, Cyathophyllum dianthus in Wenlock limestone, Trinucleus Caractaci in Caradoc, Ogygia Buchii in Llandeilo flags.

The results obtained by M. Barrande in the small Silurian basin of Bohemia are very similar—the same rather strong division of Upper and Lower Silurians being admissible.

In North America we have a numerous list of fossils, derived from a greater number of beds, and the general parallelism of all the deposits from the base of the carboniferous to the top of the hypozoic series, together with the gradual shading of one group of fossils into another, renders the division into Devonian, Upper Silurian, Lower Silurian, and Cambrian somewhat objectionable. Hall (Geol. of New York) admits what he supposes to be Devonians into the same system as the Upper and Lower Ludlow. This classification may be useful for reference to English geologists.