PALEONTOLOGICAL DIVISIONS. 183 



the lithological character of their sediments. For the advancement of geo- 

 logical science it is necessary for the geologist and paleontologist to agree 

 upon some broad principle governing the division of Paleozoic time and for 

 the purpose of correlating the strata of one locality with those of another. 

 From long experience it is found that a division based upon paleontological 

 data is the only one which will meet the requirements over widely separated 

 areas of the globe. Structural breaks, based upon unconformities of deposi- 

 tion or lithological distinctions, determined by manner of occurrence, may 

 meet the needs of local geological provinces far better than a paleontologi- 

 cal classification, but for broader continental areas they are far too restricted 

 for the purposes of correlation. The broad divisions at Eureka are based 

 upon paleontological evidence. In the 6,000 feet of Cambrian sediments 

 above the base of the Olenellus shale, the Lower, Middle, and Upper Cam- 

 brian horizons are all well represented by characteristic faunas. The line 

 between the Cambrian and Silurian is drawn just above the Hamburg shale, 

 and is determined wholly by faunal development. In the interstratified 

 grits and limestones, which bring in the Pogonip, animal life undergoes 

 a gradual change with the extinction of an old fauna and the coming in of 

 a new one. Without any marked physical disturbance and a continuance 

 of limestone strata, a commingling of forms is to be expected. A few of the 

 more persistent Potsdam types are found at the base of the Pogonip, but a 

 characteristic Chazy fauna rapidly takes the place of the life found below 

 the Hamburg shale. At the top of the Pogonip the Trenton epoch is fore- 

 shadowed by the presence of a number of species characteristic of that hor- 

 izon on the Atlantic border. The Eureka quartzite affords no evidence of 

 animal life, but immediately upon the renewal of conditions favorable to 

 limestone deposition several of the same Trenton species reappear, strongly 

 reinforced by a group of forms decidedly Trenton in its aspect, while by 

 far the greater part of the life observed below the quartzite has passed away 

 forever. The Trenton is followed by a monotonous limestone 2,000 feet in 

 thickness, carrying a few scattered corals, Hali/sitcx catcnulatus being suffi- 

 ciently characteristic to identify the beds as belonging to the Niagara. The 

 Silurian of Eureka consists, then, of two heavy bodies of limestone with dis- 

 tinct faunas, separated by a dense white quartzite. 



