692 SCHUCHERT AND TWENHOFEL ORDOVICIC-SILIJRIC SECTION 



Orthoceras ^ninganense, 0. cornuum, Thaleops clavifrons, Thaleops arc- 

 tura, and Bumastus trentonensis. 



This same series of limestones is seen to better advantage and with a 

 greater thickness on Bald Island, due to an upward flexure of the strata. 

 The zone Ag begins here 35 feet above the basal limestones, and is about 

 65 feet thick. It consists of coarsely granular, white limestones almost 

 chalky in spots, not well bedded below, but better stratified above. The 

 lower half is a fossil breccia with much crystalline calcite and diagenetic 

 destruction of the fossils. It has furnished, among others, the following 

 forms: Solenopora compacta, Ischadites, ^Stylarcea parva, Phylloporina 

 sublaxa (Lowville type), Bhinidictya mutahilis^ and variety major, Eri- 

 dotrypa mutabilis, ^Orthis ignicula, Hehertella cf. hellarugosa, *fl". ex- 

 foliata ?, *Camarella varians, ^RhynchoU-ema (?) prinstana, ^Rafin- 

 esquina champlainensiSf '^Leptcena incrassata (common), Maclurites 

 logani, Strophostylus cf. textilis, Holopea cf. pyrene, "^Gonocardium 

 beecheri, Oncoceras two species, Poterioceras, Cyrtocerina cf. typica, 

 8pyroceras, Cameroceras, Leperditia, Leper-ditella, Bythocypris, *Bu- 

 mastus erastusi, *B. globosus, *Amphilichas minganensis, "^Ceraurus 

 pompilius, Bathyurellus brevispinus, Glaphurus, and "^ Pseudo splicer exo- 

 chusvulcanus ?. 



Zone Ag is closed by a fine grained, dense, whitish limestone 28 feet 

 thick, with bands of granular limestone. These beds are not well ex- 

 posed on Bald Island, but on a smaller island immediately to the south- 

 west they can be seen to advantage. The only fossils gathered are large 

 Leperditia. 



Correlations. — The few species known from the introductory Mingan 

 zones Ai and Ag link them directly with zone A3. Their strata represent 

 the near shore sediments of the invading sea and in the basal quartz con- 

 glomerate the brachiopods consist of separated valves that also show wear 

 due to wave action. The fauna of A3 is abundant, and begins with a 

 profusion of Rhynchotrema (?) orientalis. When this biota is all deter- 

 mined it may attain to 60 species, and at first sight gives one the impres- 

 sion that it is of middle Chazian time. This is due to the presence of 

 many Chazian species (see the list on pages 690-1), and particularly of 

 Stylarcea parva, Bolboporites, Leptcena incrassata, Camarella varians, C. 

 longirostra, Maclurites atlanticus, Bumastus globosus, and Pliomerops 

 canadensis. On the other hand, an abundance of bryozoa (rare in the 

 lower and middle Chazy), the sponge Zittelella varians, and the brachio- 

 pods Orthis ignicula and Hehertella exfoliata ? give the fauna an early 

 Black Eiver aspect. It was this Mohawkian impress that led the writers 

 to believe these lower zones of the Mingan series to be not older than 



