RELATIONS BETWEEN DEVONIC AND SILURIC STRATA 355 



The identification of the other fossils with Lower Helderberg species 

 of New York may, perhaps, also be questioned, since the imperfect 

 preservation makes absolute determination almost impossible. Even if 

 the determination should be correct, the weight of evidence is in favor 

 of the Manlius limestone rather than Lower Helderberg age of the strata 

 containing these remains. The Lower Helderberg series of rocks, as 

 well as some of the other Devonic beds of New York, are unrepre- 

 sented in these districts. 



A striking piece of evidence of a pronounced unconformity between 

 the Siluric and Devonic strata in northeastern Illinois has recently 

 been described by Weller (1899). He found at the village of Elmhurst 

 that an enlarged joint fissure in the Niagara limestone was " filled with 

 a breccia composed of angular fragments of the adjacent limestone im- 

 bedded in a dark brown matrix." This matrix contains an immense 

 number of fish teeth and brachiopods, which indicate the Upper Devo- 

 nian age of the deposit. Weller deduces from this evidence that " during 

 the greater part of Devonian time the region now known as northern 

 Illinois was above sealevel.' 1 He holds that the natural joint fissure was 

 widened through solution by meteoric waters, and that at a later period, 

 near the close of the Devonic, the area was reoccupied by the sea, and 

 the sand and organic remains sifted down into the joint fissure. 



The Contact in Erie County 



unconformity 



The upper surface of the Manlius limestone of Erie county is knotty 

 and concretionary, producing minor irregularities. There are also dis- 

 tinct and well marked traces of erosion of these strata prior to the depo- 

 sition of the overlying rock. These can be seen in the various natural 

 exposures, but are usually best shown in the quarries. Hall * says that — 



" The surface on which the higher limestones rest is very uneven, consisting of 

 abrupt elevations and depressions very similar to the channeled bed of a powerful 

 stream." 



The following figures illustrate some of the features of the contact as 

 exposed in the cement quarries in North Buffalo. Figure 1 represents 

 a gentle sag in the beds of the Manlius limestone, an uneven erosion 

 plane truncating the ends obliquely. On this uneven surface rests the 

 Onondaga limestone, which thus has an unconformable relation to the 

 Manlius beds. The length of the section represented is about 10 feet. 



* Hall, 1843, p. 146. 

 LT— Bult,. Geot,. Son. Am., Vol. 11, 1899 



