CHARACTER AND DISTRIBUTION 949 



the beds which mark tlie inaugnration of Sherburne conditions of sedi- 

 mentation. 



East of Smyrna the Hamilton is succeeded conformably by the Sher- 

 burne sandstone, which carries only plant remains and which apparently 

 originated as a low bar, which at the beginning of Upper Devonic 

 time divided the Tully sea from a water body to the west in which the 

 Ithaca fauna developed.^ It is thus evident that the Tully sediment could 

 not have been derived from the east any more than it could have been 

 derived from the west, where lay the shallow pools in which the iron 

 pyrite layer was forming ; but no such objection applies to the north.. 

 where the former continuation of this rock has been removed by erosion. 



Eelationship of the Tully Limestone and Genesee Shale 



■ The Genesee black shale everywhere succeeds the Tully limestone. It 

 is 75 feet thick at Tully,"* where it seems to succeed the limestone some- 

 what abruptly, though no actual contact has been observed. It is thence 

 traceable eastward to about the meridian of Smyrna, in Chenango County, 

 or as far as the Tully limestone is developed, beyond whicli point it dis- 

 appears and is replaced by tlie sands of the Sherburne formation. The 

 abrupt change from the shale to the sandstone precludes the possibility 

 that the muds of the Genesee were derived from this eastern region. 

 These relations are shown in figure 1, page 950, which represents an 

 east-west section through N'ew York State. As we have seen, the Genesee 

 shale thins regularly westward, until in eastern Erie County it is only 

 about two feet thick and at Eighteen Mile Creek less than an inch. Evi- 

 dently the source of the black mud could not have been in the west. It 

 could not have been to the north, if that was the source of the limestone 

 mud of the Tully, and hence we must regard the south as the only pos- 

 sible source of the deposit. This is indeed shown to be the case by the 

 gradual thickening of the Genesee southward, v^ith a corresponding de- 

 crease in the thickness of the Tully limestone which underlies it. The 

 section of the Tully and the Genesee exposed on Cayuga Lake, a few miles 

 north of Ithaca, is extremely instructive in this connection. Through the 

 kindness of Prof. G. D. Harris, I was enabled to study this section in some 

 detail. The total thickness of the Genesee is here 135 feet, and it is 

 capped by an arenaceo-calcareous layer from one to two feet thick, which 

 contains many pyrite concretions. This probably represents the horizon 

 of the Genundewa limestone, though it is generally not composed of Sty- 



s The Sherburne problem is discussed in another paper. 



