210 INDEX TO THE STEATIGRAPHY OF NOETH AMEEICA. 



them occurs a yellowish-white pure limestone, 30 feet thick, carrying Maclurea 

 logani, which Logan ^^^° refers to the "Birdseye" (Lowville) and Black River. 



A detailed section of the strata exposed above the beach at the west end of 

 Anticosti is given by Logan, ^^^ with much refinement of distinction among the 

 successive beds and lists of fossils found in each. Limestones and argillaceous 

 shales in thin layers make up most of the thickness noted (959 feet). Gray lime- 

 stone, greenish shale, and a few layers of limestone conglomerate alternate in the 

 lower 229 feet; bluish and reddish-gray limestone separated by thin greenish 

 shale partings and conglomerates succeed with slight variations throughout the 

 upper 730 feet. 



The Ordovician here passes without notable lithologic change or break into the 

 Silurian, and a number of fossils are common to strata above and below the assumed 

 plane of division, yet numerically the extinction of species is more marked than 

 their persistence. Billings *^ states the facts as follows : 



In the Lower Silurian rocks of Anticosti there have been collected 121 species of fossils, 

 of which the proportionally large number of 85 have been described in this and other pubhca- 

 tions of the Survey as new forms. The remaining 36 are mostly of the common and widely 

 distributed species of the Lower Silurian of Canada West, New York, and other countries. 

 They are the following: 



Stenopora fibrosa. 

 S. mammulatfa. 

 S. papillata. 

 S. explanata. 

 Halysites catenulatus. 

 Lingula quadrata. 

 Trematis ottawaensis. 

 Strophomena imbrex. 

 S. subtenta. 

 S. plamimbona. 

 S. altemata. 

 Leptsena sericea. 

 Orthis testudinaria. 

 O. subquadrata. 

 O. lynx. 



Rhynchonella capax. 

 R. recurvirostra. 

 Ambonyclda radiata. 



Subulites ricbardsoni: 

 Trochonema umnilicata. 

 Pleurotomaria americana. 

 P. belena. 

 P. Bubcomca. 

 Murchisonia gracilis. 

 M. ventricosa. 

 Belleropbon acuta. 

 B. bilobatus. 

 Pterotheca transversa. 

 Oncoceras constrictum. 

 Asaphus platycephaluB. 

 A. megistos. 



Dalmanites caUicepbalus. 

 Cheirurus pleurexanthemus. 

 Harpes ottawaensis. 

 Calymene blumenbachi. 

 Leperditia canadensis. 



There are np species which are exclusively Upper Silurian; the aspect of the whole fauna 

 is eminently Lower Silurian. The rocks are very fossOiferous throughout, but on approaching 

 the dividing line between this group and division 1 of the Anticosti group, which immediately 

 succeeds, not less than 80 out of the 121 species suddenly disappear and are seen no more. 

 It is evident, therefore, that there is here a break of considerable importance, probably in some 

 way connected with the great gap that occurs between the Hudson Eiver and Clinton forma- 

 tions in Canada West and New York. Of the 41 species that pass this break, 30 appear to 

 have become extinct during the period of division 1 ; at least they have not been detected in 

 division 2. Of the remaining 11 species, 7 pass upward into division 3 and 6 into division 4. 



Ulrich comments to the effect that according to present understanding of the 

 Anticosti problem, none of the beds in Logan's section is older than Richmond. 



