116 CLASSIFICATION OF THE NEW-YORK ROCKS. 



This important division may be farther subdivided into four groups : 1. Medina sandstone, 

 consisting of hard and soft bands of rocks, sometimes suitable for flagging. 2. Green shales, 

 oolitic iron, thin beds of limestone, and coarse and fine grits. 3. Niagara limestone. 4. Red 

 and green calcareous shales, with the thin beds of limestone which together form the On- 

 ondaga salt group. 



3. The Helderberg division, comprising, 



1. The Manlius waterlimes and thin shales. 



2. Pentamerus limestone. 



3. Delthyris shaly limestone. 



4. Encrinal limestone. 



5. Oriskany faiidstone. 



6. Cauda-galli grit. 



7. Schoharie grit. 



8. Onondaga limestone. 



It is difiicult to subdivide this group by neutral planes, though the objection to consider it 

 under the three following divisions are not very great : 1, the Waterlime group, em- 

 bracing the thin and lowest beds, and the rocks up to the Oriskany sandstone ; 2, the 

 Sandstone group, embracing the Oriskany sandstone, Cauda-galli and Schoharie grits ; 

 3, Onondaga limestone, including the Selenurus rock of Conrad, 



4. The Erie division. It embraces the following rocks : . . ' 



1. Marcellus shales, terminating in a hydraulic dark-colored limestone. 



2. Shales and grits of great thickness, which have been denominated the Hamilton group. 



3. Shales and grits alternating in thin beds, which, taken together, have received the appellation of 



Chemung and Portage groups. 



The upper beds are extremely deficient in limestone ; only thin bands existing, which seem 

 to have derived their origin from the fossil shells they embrace. This upper division, 

 which is intended to extend to the Catskill or Old Red Sandstone, is distinct and correct so 

 far as lithological characters are concerned, and may be undoubtedly subdivided by means 

 of fossils. It embraces without doubt the Devonian system of Phillips ; but as yet it is 

 quite difficult, if not impossible, to say where this system begins. The change from the 

 Marcellus shales upward to the Catskill rocks, is so gradual and imperceptible, that the 

 outgoing of the Silurian or New-York system, and the incoming of the Devonian, never can 

 be settled by geologists, except by conventionally agreeing where the one shall stop and 

 the other begin. 



5. The Catskill division, or the Old Red system. It is formed by the green and cho- 

 colate grits, and sometimes by a red marl, a material softer than a sandstone usually is. 

 This division, although it has been denominated the Old Red sandstone, is made up of by 

 far a greater amount of greenish-colored grits than of red ones. Beds of conglomerate 

 also abound in difTercnt parts of the mass, but more conspicuously towards the top of tlie 

 Catskill mountains. 



