GEOLOGIC FOEMATIONS OF NEW TOEK 157 



lower strata are traceable in the Appalachians as far as Tennes- 

 see, though their upper limestones do not extend beyond the 

 Susquehanna. In following them westward from Albany county, 

 we find the lower limestones and sandstones thin out rapidly, 

 not extending beyond the Niagara in any considerable thickness, 

 while the upper limestones are found spreading into the far west. 

 This series of rocks which may be considered collectively in its 

 effect on topography, belongs partly to the Upper Silurian sys- 

 tem and partly to the Devonian and may be divided into two 

 parts; the Lower Helderberg limestones which are of Upper Silu- 

 rian age, and the Oriskany sandstone and Upper Helderberg lime- 

 stones which are included in the Devonian. 



Lower Helderberg Group 



The subdivisions of this group are as follows: 



Thickness 



Scutella limestone ) 15 ft in Albany county 



Upper Pentamerus ) 



Delthyris, or Catskill shaly 



limestone 100 ft. 



Lower Pentamerus limestone 65 ft. in Albany county 

 Tentaculite limestone 30 ft. " 



The Scutella limestone, named from a fossil crinoid which it 

 contains, is the uppermost member of the group where it occurs, 

 but it has not been found associated with the Upper Pentamerus. 



The Lower Pentamerus limestone is coarse-grained, thick- 

 bedded and often a concretionary limestone; while the Catskill 

 limestone is in thin layers, with much shaly or slaty matter in- 

 terstratified with it. 



The Lower Helderberg group has its greatest development in 

 Albany and Schoharie counties; the subdivisions above given 

 may be differentiated in Greene, Albany and Schoharie counties, 

 but west of the last county they are not distinct and the group 

 itself is indistinguishable from the Salina formation, at the sur- 

 face, west of Seneca lake. In the Livonia salt shaft, however, 

 about 35 feet of limestone was found containing Tentaculite fos- 



