Luther — Geology of the Salt District. 



223 



Portage Group. The next formation in upward order of succession is 

 the Portage group, which is composed of light and dark shales, thin flags and 

 heavier bedded sandstones and has an aggregate thickness in the eastern part 

 of the salt district of 1200 to 1300 feet, and 800 to 1000 feet in the Genesee 

 and Oatka valleys. 



It constitutes the surface rock over a considerable portion of the territory 

 in which the producing wells of the salt district are located. 



In the eastern part of the district the lower beds are mainly light colored 

 sandy shales or flaggy sandstones, but include also strata of finer and softer 

 fissile shales, very dark brown or black in color. 



At Ithaca two layers of sandstones, together about four feet thick, are at 

 the base of the group and above them are alternate strata of shales and sand- 

 stones aggregating 1300 feet in thickness that are considered on paleontological 

 grounds to have been deposited in Portage time. A part of these beds 250 

 feet thick, the bottom of which is about 350 feet above the top of the 

 Genesee, contains an association of fossils entirely different from that found 

 in the remaining Portage beds above and below it. This subdivision is well 

 defined about the head of Cayuga lake and eastward in Cortland and 

 Chenango counties it thickens to such a degree as to represent almost com- 

 pletely the sedimentation of Portage time. 



These beds were termed by Professors Hall and Vanuxeni, the Ithaca group. 

 Its limits are obscured on Seneca lake and have not been clearly traced west of it. 



In the Naples valley at the head of Canandaigua lake, the shales are 

 softer and generally lighter colored, and the sandstones thinner than in the 

 localities last mentioned, and the outlining of the divisions, so distinctly 

 marked in the Genesee river section, is begun. 



'At the base of the group there are 230 feet of soft olive and light blue 

 shales, in which are intercalated a few thin sandstones. The lower part of 

 these beds is quite barren, but fossils are common in the upper part. 



Twenty-two feet of densely black fissile shale separates these Cashaqua 

 beds from the darker and harder shales, and thin, hard, blue sandstone flags, 

 that correspond to the Gardeau flags and shales of the Genesee river, and 

 which have an aggregate thickness of 450 feet. 



At the top of the group the sandstones are fairly well developed, 

 consisting of layers of blue sandstone one to three feet thick separated by a 

 few inches of hard shale, and having an aggregate thickness of nearly fifty 

 feet. If these sandstones are coextensive with the original Portage sandstones 

 of the Genesee valley, the thickness of the entire Portage group is approxi- 

 mately 050 feet in the Naples valley. 



