160 W. H. Twenhofel — Silurian Section at Arisaig. 



show that the dark green rock overlying the red shale is a 

 volcanic breccia, containing, in addition to the angular frag- 

 ments of glass and fine material, pieces of quartz and feldspar. 

 Small flakes of clay are also present, which appear to be due 

 to alteration subsequent to deposition. 



Correlations by Charles Schuchert. 



Mr. Twenhofel gathered a large collection of the Arisaig 

 fossils in over 200 lots from as many horizons in the section of 

 3465 feet. In looking over this mass of material one is 

 impressed with the strangeness of this Silurian fauna, which is 

 more European in derivation than American. None of the 

 characteristic Silurian fossils of the United States is present 

 excepting a few forms like Anoplotheca hemispherica, Cam- 

 arotoechia neglecta, C. ohtusiplicata, and a few other brachio- 

 pods occurring in all Silurian districts. On the other hand, 

 the pelecypods remind decidedly of Westmoreland, England, 

 and yet the widely distributed European Cardiola interrupto, is 

 not present, but is said to occur in northern Maine. Another 

 peculiarity of this Silurian fauna is the almost complete absence 

 of corals, but this fact may be ascribed to the muddy and sandy 

 shore condition of this sea. The only American Silurian 

 region having a fauna suggesting direct marine connections is 

 that of the Appalachian trough, best known about Cumberland, 

 Maryland. * A comparison with this region shows, however, 

 that the Arisaig fauna has its own decided characteristics ; 

 but that it still has by far more in common with the Cum- 

 berland trough extending to Clinton in eastern New York, 

 than with the faunas of western New York or the Mississippi 

 Valley. Another striking fact is that even though the Arisaig 

 section is a very thick one and though it has been stated more 

 than once that the Helderbergian faunas or their equivalents 

 are present, there are no fossils in these strata suggesting 

 anything more recent than the Ludlow (of the Christiania 

 area of Norway). 



Division I. — This horizon is usually regarded as equivalent 

 to the Medina of New York ; but on grounds other than its 

 position, there is not the slightest evidence for this correlation. 

 As yet no Anoplotheca hemispherica have been gathered here, 

 but the other fossils are those of Division II. None of the 

 typical Medina fossils is present and until such are found these 

 strata are best regarded as of Clinton or Lower Landovery 

 time. 



Division II or Arisaig formation. — The lowest faunas of 

 the Arasaig series are distinctly Silurian and there is nothing 

 present representing the thick transition series between the 



*See Prouty, this Journal, Dec, 1908, pp. 553-574. 



