32 THE GEOLOGIST'S TRAVELING HAND -BOOK. 



greatest abundance. So rare is the occurrence of regular layers in the group, 

 that their absence is a good negative character, and its brownish or yellowish 

 color, externally, or where weathered, a good positive one of the group generally. 

 This applies to the central, but not to the eastern part of the State of New York. 

 It abounds in fossils, and is admirably characterized by them, numerous species and 

 even genera commencing with the group, and ending with it. Van TL 150. 



In the western part of the State of New York, instead of sandy shale and 

 shaly sandstone, and even tolerably pure sandstone, as in the east, the sand has 

 diminished and the clay increased. The group, as a whole, presents an immense 

 development of dull olive, bluish-gray calcareous shales, which, on weathering, 

 assume a light gray or ashen tint, some thin portions becoming brownish on 

 exposure. The formation thins out very much in going westward, and at Lake 

 Erie has only half the thickness found at Seneca Lake, and is so different that 

 doubt of the identity of the two might arise, if one judged by the appearance only. 

 The Hamilton is the New York lake formation, the following lakes being excavated 

 in it: Otsego, Cazenovia, Skaneateles, Otisco, Owasco, Cayuga, Seneca, Canandaigua, 

 and the north end of Hemlock Lake. The east end of Lake Erie is also cut out of 

 the Hamilton. The upper part of the Hamilton was called the Moscow shale, 

 from a place between Mt. Morris and Rochester, on the Genesee River. 



In Pennsylvania the Hamilton shale has been measured on the Juniata, 635 

 feet thick. It has many hundreds of miles of outcrop, in repeated zig-zags, form- 

 ing, in combination with the Genesee and Portage above it, ranges of smooth, 

 cultivated hills, of an entirely characteristic shape, in long lines of ruffled slopes, 

 regularly indented with short and smooth ravines. This striking topographical 

 feature, maintains itself throughout the mountain-region into Virginia, and still 

 farther south. The abundance of shells, without limestone beds, in Pennsylvania, 

 furnishes a partial clue to the deposit of the (next succeeding) Tully limestone in 

 New York. 



10 b. Tully Limestone. This is the dividing line, easy to find, between the 

 Hamilton and Genesee, being the upper part of the former, and it is important in 

 New York as the most southern mass of limestone in the State. It is only local, 

 and is an impure limestone, fine-grained, usually a dark or blackish blue, often 

 brownish. The usual thickness of the rock is about fourteen feet, and its greatest 

 thickness twenty feet. It makes a good but not a white lime. It receives its name 

 from the township of Tully, in Onondaga County, New York. This limestone 

 often shows an accretionary structure, and a roughed, notched appearance, where 

 its layers separate as in some of the layers of the water-lime. One of the lower 

 layers is thick, the bottom one being frequently five feet in thickness, and it is 

 owing to this circumstance, and to the softness of the shale beneath, that when- 

 ever a waterfall exists, the shale has been washed out to some depth, leaving a 

 chamber or cavern, of which the limestone forms the roof or ceiling. V. 169. 

 It is a marked geological horizon in Central New York, being the termination 

 of the Hamilton, and is succeeded by shales of a widely different character. 

 It is often thick-bedded, but it is often divided by numerous irregular seams into 

 small fragments. Its color, on first exposure, is blue or nearly black, but weathers 

 to an ashen hue. It is best seen on the Cayuga Southern R. R., where it stands out 

 in the face of the cliffs as a prominent band. It is absent west of Canandaigua 

 Lake and in the eastern part of the state. H. 212. 



