68 [Assembly 



The Hamilton group terminates in Central New-York with a 

 very impure dark limestone, about ten feet thick, called the 

 TuLLY LIMESTONE. lu the castcm and western parts of the 

 State this rock does not exist, and the Genesee slate lies directly 

 on the Hamilton group. The Tully limestone contains some fos- 

 sils which are common to it and lower shales, among which are 

 the Phacops hvfo and Dalmania calliteles (see page 65) ; and some 

 peculiar to itself, the most marked of which is a beautiful bra- 

 chiopodous shell, the Rhynchonella cuboides^ shown in Fig. 35, 

 No. 1. Beside the fossils in this cut, there are others ; among 

 them, a very pretty small Lept^na. 



The next rock in upward order is the Genesee slate, a series 

 of layers of thin-bedded fissile black slate, in some places 150 

 feet thick, but diminishing westward so that it is only about 25 

 feet on Lake Erie. It is, however, distinctly recognized in Penn- 

 sylvania, where it is some 300 feet thick, and, with the Marcel- 

 lus slate and Hamilton group, forms the " Cadent group" of 

 Prof. Rogers' Report. It derives its name from one of its best 

 localities in this State, the gorge of the Genesee below Portage. 

 It is generally easily recognized by its black soft slaty texture, 

 but its fossils are very few ; the more common forms being shown 

 in the woodcut Fig. 36. 



The name of 



PORTAGE GROUP 



has been given to the next upward portion of the great slaty and 

 shaly masses, which form the deep gorge of the Genesee at Por- 

 tage, and everywhere cover the southern edges of the Hamilton 

 group and Genesee slates. This enormous pile of sandy, slaty, 

 and shaly strata is in some parts of the State 1000 feet in thick- 

 ness : it is divided^ in Prof. Hall's Report on the Fourth District, 

 into a lower mass called the Cashaqua shale, a middle mass called 

 the Gardeau shale and flagstones, and a terminal mass of sand- 

 stones seen at Portage ; but in Middle and Eastern New- York, 

 these divisions are not distinct. 



Much of this group is a soft olive-colored shale ; but its most 

 useful portions are its layers of flagstone, which are largely 

 quarried near Norwich and Ithaca, on the hills back of the Hel- 

 derbergs, and on those west of the Hudson river as far down as 

 Rondout. Layers of the same character exist in this group in 

 Pennsylvania, where it attains an enormous development, being 



