50 F. H. McLearn — Lower Ordovician 



Johannian, a thick series of cross-bedded sandstones, with 

 worm burrows and broken Lingulellas ; the age is probably 

 Upper Cambrian. On the sandstones lie the black slates of 

 the Bretonian, which Matthew divides into four zones. 

 Diciyonema flahelliforme Eichwald appears in the second 

 band, b, and attains its full development in the band c. The 

 fourth zone, d, contains a Tetragraptus-Didymograptus fauna, 

 a consideration of which is the object of the present paper. 



Location. — Our knowledge of this fauna dates from 1891, 

 when Matthew found graptolites in the black slates at the 

 Suspension Bridge, just below the Reversing Falls. It is 

 interesting to note that this is the only known outcrop of the 

 zone in the Maritime Provinces (which of course excludes 

 Quebec localities). The fossils occur in nearly vertical beds on 

 the north shore of St. John harbor, a short distance below the 

 bridge. 



Lithology. — The rock is a black, fissile, very fine-grained, 

 carbonaceous slate. It is very slightly calcareous, due in part 

 to . the presence of articulate brachiopods. Matthew (1892J) 

 mentions the presence of calcareous nodules, but none have 

 been seen by the writer. Pyrite occurs in joint planes, 

 replacing the orthoids in part and occasionally the graptolites. 



The weathered rock cleaves into paper-thin layers, and, owing 

 to the fact that the bedding and the cleavage coincide, the 

 graptolites are preserved in the surfaces of these layers. 

 Gypsum is present in the cleavage planes in very small crystals. 



Structural Relations. — About 35 feet of the slates are 

 exposed in the cliff, below the bridge, where they dip to the 

 south at a high angle under the waters of the harbor. On the 

 north, the formation is bounded by a fault (Matthew 1891, p. 128; 

 Young 1913, p. 386) which brings it against the Johannian 

 sandstones and thus obscures the relationship of the Tetragraptus 

 zone to the older formations. It follows that both the upper 

 and lower limits of the Tetragraptus zone at St. John are 

 unknown, and the field relations contribute but little to the 

 determination of their age. 



The structure, at this locality, has never been clearly 

 defined. Matthew (1891, p. 128) states that "in this section 

 (at the western end of the city) the whole series of the St. John 

 Group is overturned, and the oldest beds appear uppermost." 

 In a figure (1891, p. 128) the beds of division 2>d are shown to 

 be overturned. This author, however, notes that the Johannian 

 beds north of the fault " are not overturned, but belong to the 

 northern side of a syncline." Young (p. 386) considers that 

 " the Ordovician measures are the highest preserved members 

 of an overturned syncline, " but does not define this syncline 



