HAMILTON GROUP. 61 



division is scarcely worthy of recognition. The three have a thickness varying 

 from 300 to 900 feet, extend from Lake Erie to the Hudson, and abound in 

 fossils. The Tully limestone was named from Tully, where it is burnt for lime, and 

 has a thickness of 14 to 20 feet. The Genesee slate, named from the opening of 

 the gorge of the Genesee River at Mount Morris, where it is a black, argillaceous 

 fissile mass, attains a thickness of 150 feet and closes the era of the Hamilton 

 Group in New York. 



§ 127. The Group extends from the Hudson to Lake Erie, occupying a belt 

 of variable width in the central part of the State, and attaining a maximum 

 thickness in the eastern part of 1,200 to 1,400 feet, and diminishing to about 300 

 feet in the western part. The valleys of Seneca and Cayuga Lakes are 

 excavated for more than half their length in these rocks, and the banks and ravines 

 afford the best facilities for examination. It is an olive shale, with slates and sand- 

 stones in the eastern, and calcareous shale and limestone in the western part of the 

 State. The bedded rocks are remarkable for the abundance of ripple-marks, and 

 wave-lines, and the shales abound in carbonaceous material, due to vegetation. 

 Fucoids and marine plants are common, and coniferous trees and ferns grew to a 

 good size, and drifted into the ocean, where they were imbedded and preserved, so 

 as to show much of their form and structure. The New York subdivisions are lost 

 in the extension across the peninsula of Canada from Lake Erie to Lake Huron, 

 and the Group becomes a limestone in Michigan. It occurs at only one place in 

 Wisconsin, which consists of a strip about 10 miles long and 5 or 6 wide, near 

 Milwaukee, where it is an impure limestone, quite fossiliferous, and largely mined for 

 the manufacture of hydraulic cement. It occurs in Ohio, resting on the Upper 

 Helderberg as far south as Columbus, and the upper part of the limestone at the 

 Falls of the Ohio, is referred to it. It occurs at Davenport and New Buffalo, in 

 Iowa, and also in Illinois and Missouri. It appears among the western mountains, 

 on the Mackenzie River, in Alaska, and in the Arctic regions. It has greater 

 thickness in Pennsylvania, New Jersey, Virginia, and other States in the Appa- 

 lachian chain, than it has in the West, and contains much more mechanical sedi- 

 ment. In the East it is a mud rock supplied with drift materials and marine 

 remains, while more westerly it is exclusively a marine calcareous rock. 



§ 128. It is of quite general distribution and usually readily determined by 

 its invertebrate fossils, which exceed in number almost all earlier Groups. 

 Lepidode)idron, which became so common in the Coal Measures, is found in the 

 shales. The remains of fish are much like those of the Upper Helderberg, though 

 species are distinct. The characteristic fossils, and those by which the Group may 

 usually be determined, are Heliophyllum halli, Spiri/era pemuita, S. granulifera, 

 Tropidoleptus carinatus, Rhyncfwnella venustula, Athyris spiriferoides, Leiorhynchus 

 limitare, L. quadricostatum, Orthonota andidata, Gypricardella betthstriata, Cimitarw 

 recurva, Pterinea flabellum, Modiomorplia concentrica, Bellerophon patulus , Pkurotomaria 

 sulcomarginata, Styliola fissurella, Homalonotus dekayi, and Phacops bufo. 



§ 129. The oil-springs of Enniskillen and of the Thames, in Canada, were 

 known to the Indians and to the settlers from an early period. The oil floated 

 upon the surface of the waters, and formed by its drying beds of tarry bitumen. On 

 sinking through the clay from 40 to 60 feet, a bed of gravel is reached, from which 

 considerable supplies of petroleum are obtained. Such are called surface-wells, 



