412 CAMBRIAN AND SILURIAN IN NORTH AMERICA. [XV. 



The former lias, moreover, carefully compared this fauna with 

 that of the lower members of the New York system, in 

 which the succession of organic life appears to have been 

 very much interrupted. Thus, according to Mr. Billings, of 

 the ninety species known to exist in the Chazy limestone 

 of the Ottawa basin, only twenty-two species have been ob- 

 served to pass up into the directly overlying Birdseye and 

 Black Eiver limestones, which form the lower part of the 

 Trenton group. The break in the succession between the 

 Chazy and the underlying Calciferous sand-rock in this region 

 is still more complete ; since, according to the same authority, 

 of forty-four species in the latter only two pass up into the 

 Chazy limestone. This latter break appears to be filled, in the 

 region to the eastward of the Ottawa basin, by the Levis 

 limestone ; which has been studied near Quebec, and also 

 near Phillipsburg, not far from the outlet of Lake Champlain. 

 This formation (including the accompanying graptolitic shales) 

 has yielded, up to the present time, two hundred and nineteen 

 species of organic remains (comprising seventy-four of crus- 

 tacea and fifty-one of graptolitidise), none of which, according 

 Mr. Billings, have been found either in the Potsdam or in the 

 Birdseye and Black River limestone. Twelve of the species 

 of the Levis formation are, however, met with in the Calcifer- 

 ous, and five in the Chazy of the Ottawa basin, and the Levis 

 is therefore regarded by Mr. Billings as the connecting link 

 between these two formations. 



With regard to the British equivalents of these rocks, the 

 Levis limestone, according to Salter, corresponds to the Tre- 

 madoc beds ; although the species of Dikellocephalus found in 

 the Levis rocks are by him compared with those found in the 

 Upper Lingula flags or Dolgelly beds. The graptolitic strata 

 of Levis, however, clearly represent the Arenig rocks of North 

 Wales, the Skiddaw group of Sedgwick in Cumberland, the 

 graptolitic beds which in Esthonia, according to Schmidt, are 

 found below the orthoceratite -limestones (Can. Naturalist (1), 

 VI. 345) and those of Victoria in Australia (Mem. Geol. Sur., 

 III. Part II. 255, 304). In the Arenig and Upper tremadoc 



