48 UPPER SILURIAN MEDINA GROUP. 



CHAPTER XII. 



UPPER SILURIAN. 



97. ALL the rocks of the Upper Silurian System are marine ; but land- 

 plants, or such as may have existed in marshes, and received support from sunlight 

 and air, have been found within them. No remains of land or fresh-water animals, 

 or marine vertebrates, have been discovered in North America. There is no radical 

 difference in the general character of the Lower Silurian and Upper Silurian fossils, 

 because vertebrates had not made their appearance, and the same orders of inver- 

 tebrates were represented in each era; but the separation into two Systems is very 

 convenient, because both are introduced with sandstone Groups, and the Trenton 

 in the Lower Silurian, and Niagara in the Upper Silurian, are alike extensive in 

 geographical distribution, and some analogy may be traced between the upper 

 Groups in each System. On the whole, the calling of one System Lower Silurian, 

 and the other Upper Silurian, was a happy hit in nomenclature as well as correct 

 in science. 



MEDINA QROUP. 



98. THIS Group took its name from Medina, New York. The rocks were 

 described by Vanuxem in 1842, under the names Oneida Conglomerate, Gray 

 Sandstone of Oswego, and Medina Sandstone. At the typical localities they are 

 conglomerate, and gray and red sandstone. The conglomerate is hard and gritty, 

 and composed of quartz pebbles and sand so firmly cemented as to be used for 

 millstones. The sandstone is argillaceous, thinly laminated, and of red, gray, and 

 mottled colors. Where it is not fragile, but firmly cemented, it^makes a good building 

 stone, and has been largely used for paving streets, as it readily breaks into stones 

 of regulation size. The Group borders Lake Ontario on the south, and extends in 

 an east and west line of exposure about three-fourths the length of the State, and, 

 entering Canada at the Niagara River, continues to Lake Huron. In Oneida 

 and Oswego Counties the thickness is from 500 to 600 feet; at the west end of 

 Lake Ontario 614 feet, and at Lake Huron 100 feet. It thins so rapidly that few, 

 if any, traces have been discovered west of this lake. A small surface area in New 

 Jersey has a thickness of 900 feet, and a larger one in Pennsylvania has a thickness 

 of 2,500 feet. It occurs in patches among the broken ranges of the Appalachian 

 System in Maryland and other States, as far south as Tennessee ; but is unknown 

 in the Western States. 



99. The conglomerate is 500 feet thick in the Shawangunk Mountains, and 

 700 feet in the Kittatinny Valley in Pennsylvania. It graduates into the gray sand- 

 stone, and then into the red sandstone, so they can scarcely be distinguished except 

 by color; and the gray sandstone in like manner graduates into the conglomerate 

 by enlarging and increasing the number of its pebbles; so there is no reason, strati- 

 graphical or palseontological, for subdividing the Group, as was done in early work 

 on the New York Survey. It always rests unconformably upon the Hudson River 

 Group, and bears the internal evidence of having been derived from land immedi- 

 ately north and east, and of having been deposited in shallow water, subject 



