280 C. SCHUCHERT MEDINA AND CATARACT FORMATIONS 



sandy, while toward the northwest the sands at the bottom vanish first, 

 the middle portion becomes increasingly more limestone, and the upper 

 shales become more and more red and unfossiliferous toward the Mani- 

 toulins. In Ontario, where the formation is typically developed, it is 

 readily divisible on its petrologic and fauna! character into three mem- 

 bers, as follows : 



Upper Cataract or Cahots Head shale member. — This was first named 

 Kagawong by Williams f but as this name had been used by Foerste for 

 an Ordovicic formation, Williams now proposes to use the term Cabots 

 Head, as given by Grabau.* The member consists of a series of shales, 

 usually greenish in color and somewhat calcareous, with occasional thin 

 beds of magnesian limestone; toward the northwest the top of this mem- 

 ber becomes more and more red or locally ferruginous, and finally is com- 

 pletely transformed into soft, red, almost unfossiliferous shales, which 

 are in color much like Queenston, though less sandy. The thickness is 

 variable, between 20 feet (due to erosion of the top before being covered 

 by the Lockport) and 75 feet (where the shales are thickest the lime- 

 stones below are thinnest). 



These shales are locally rich in bryozoans, and particularly in the fer- 

 ruginous zones near the top of the member, where there is also more or 

 less of magnesian lime material. Here Ilelopora fragilis, Pachydictya 

 crassa, Pho'nopora explanaia, P. euMformis, CaJlopora magnopora, and 

 Trepostomata Bryozoa abound, especially at Limehouse and Dundas. 

 Other fossils, except Lingula clintoni {ohlonga of local collectors) and 

 Pterinea ? primigenia, are very scarce. Wherever a thin limestone is 

 present it is usually seen to be made up of one or two species of Helopora. 



Middle Cataract or ManitouUn limestone member (Williams, 1913). ''• — 

 Strata variable in character from heavy-bedded, somewhat magnesian 

 limestones, with local reefs of corals and bryozoans, to thinner bedded, 

 highly magnesian, and more or less impure limestones. Locally in the 

 south there are even thin beds of sandstone. The thickness varies be- 

 tween 60 feet in the north and 9 feet (Dundas), or even nothing in the 

 south (Niagara). In other words, the limestones are translated more 

 and more into shale southeastward. 



This division is nearly always rich in fossils and is the home of most 

 of the Cataract forms other than the bryozoans, though many of these 

 are also to be had here. The guide fossils are Clatlirodictyon vesiculo- 

 sum., Acervularia (?) gracilis, DipliypJiyllum vennori, Rhinopora verrii- 



3 Ottawa Nat, vol. 27, 1913, p. 37. 



< Bull. Geol. Soc. Am., vol. 24, 1913, p. 



5 Op. cit., p. 38. 



