PALEONTOLOGICAL DIVISIONS. 183 



the lithological (•haraoter of theii- sediments. For the advancement of geo- 

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

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

 the pnrpose of coirelating 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 Avidely separated 

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

 tion or lithological distinctions, determined by manner of occuirence, 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 coiTelation. The broad divisions at Eureka are based 

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

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

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

 between the Cambrian and Silurian is (b-awn just above the Hamburg shale, 

 and is determined wholly by faunal de\elopment. In the iuterstratified 

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

 a gradual change with the extinction of an old tauna 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 quaitzite aflfords no evidence of 

 animal life, but innnediately 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 gi-eater part of the life obseiwed 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, Halysites catenulatus 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 vnXh. dis- 

 tinct faunas, separated by a dense white quartzite. 



