FOSSIL REEFS: MISSISSIPPIC 431 



inclined marginal strata have been noted in a number of sections 

 in the Eifel, and the character of the reefs is not unlike that of 

 the more massive xA.merican reefs, such as that of the Niagaran of 

 Wisconsin. The reef masses rest upon a foundation of crinoidal 

 limestone. 



The corresponding ^Middle Devonic Calcaire de Givet (Givetien) 

 of Belgium has the reef structure well developed. (Dupont-27.) 

 The reefs have at first impression the appearance of amorphous 

 limestone, passing into more or less saccharoidal rock. On weath- 

 ered surfaces a somewhat brecciated structure appears, together 

 with numerous outlines of corals or sponges, slightly left in relief 

 by the solution of the enclosing limestone. Stromatoporoids make 

 up a large part of the reef, but they have been profoundly altered, 

 so that it is difficult now to separate the stromatoporoids along their 

 laminae or structure planes even with a hammer. It is only on 

 weathered surfaces that the true composition of the reef is shown, 

 the fresh surfaces showing only a compact rock often with con- 

 choidal fracture, full of cavities, but without trace of the stroma- 

 toporoids or other organic content. As in other cases, the reef is 

 structureless, no stratification being visible. The reef-formers be- 

 side the stromatoporoids (Stromatactis and Pachystroma), include 

 Favosites, Alveolites and more rarely cyathophylloids. CyatJio- 

 phyllnm cccspitosnm occurs on the margin of some of the reefs in 

 crowded heads from 1.5 to 2 meters in diameter. The flanking beds 

 of clastic limestone show well-marked stratification, and between 

 the reef knolls these clastic limestones often contain beds of crin- 

 oidal fragments, and otiiers largely composed of shells. 



Dupont deduces from the arrangement of the reef masses that 

 they constituted fringing reefs to the land of that period, and he 

 has found indications of a breach in the reef line in the Frasnien 

 reef north of the Couvin, which is analogous to the interruptions in 

 the reef opposite the mouths of streams coming from the fringed 

 land. Reef structures of similar character are found in the Devonic 

 of the Karnic Alps on the Austro-Italian border. (Frech-34; 35.) 

 The reefs are chiefly in the Middle Devonic, and composed of Alve- 

 olites, Heliolites, Stromatoporas, etc., while Stringocephalus and 

 other brachiopods are among the other characteristic organisms of 

 the reefs. The reefs themselves rest on bedded Lower Devonic 

 (Konjeprussian) limestones composed of an abundance of brachio- 

 pods and crinoids, of corals and of other organisms. 



IMississipPic Reefs. These are less abundant and are not so 

 widespread as the Devonic reefs. None have as yet been described 

 from America, though they are known to exist in some sections. 



