22 NEW YORK STATE MUSEUM 



was more fully developed in the later deposits removed by glacial 



erosion. 



Summary 



We may briefly summarize the evidence from these sections thus : 



The more prolific development of the Guelph fauna in the lower 

 Shelby dolomite on Oak Orchard creek does not extend so far eastward as 

 Monroe county and has as yet been observed only at the original locality. 



The Guelph fauna of the upper Shelby dolomite, which is largely 

 involved in chert nodules, appears under similar conditions both at Shelby, 

 about Rochester, and in the Niagara Falls section. 



It is to be noted that, while the white chert segregations are in some 

 measure an index of the upper Guelph horizon, those which contain fossils 

 have proved to be in an exceedingly small ratio to the number present. 

 The experience of Mr Arey in the exposures about Rochester showed that 

 these fossils were to be had only by very great diligence and watchfulness, 

 and it seems probable that they will always be of rarity. The dolomite 

 containing these silicious nodules weathers freely to sand, retreating from 

 about the nodules, which thus become loosened and set free, so that the 

 rough, scraggy dolomitic blocks with which the surface of the country is 

 freely covered, specially in the towns of Ogden and Sweden, Monroe co., 

 seem to us to be in part at least derived from this upper Guelph horizon. 



We conclude that the episode of the Lockport dolomites, which was 

 virtually the closing sedimentation phase of the true marine Siluric, embraces 

 representations of two quite distinct faunas ; (i) the essential or normal fauna 

 of the time and place, the immediate successor of, and derivative from the pro- 

 fuse Rochester shale (Niagaran) fauna, that is to say, the peculiar and appro- 

 priate fauna of the Lockport stage ; (2) at least two, perhaps three manifes- 

 tations of the typical Guelph fauna which has entered this province from the 

 west. These are embedded in the dolomites and interbedded with the layers 

 containing the other fauna. They represent a distinct organic facie's from 

 the other, and the relations of both are those of mutually encroaching 

 faunas of adjoining provinces without alteration of sediment and sea. 



In New York, therefore, both Lockport and Guelph faunas pertain to 

 the period of the Lockport-Shelby dolomites. 



