STRATIGRAPHICAL GEOLOGY. 11 



The Onondaga Salt Group is sometimes called the Onondaga limestone or 

 Gypsiferous series. Its outcrop in New York is traced, says Prof. Hall, from 

 Montgomery county, where the formation is represented by a thin band, West- 

 ward into Wayne county, where it attains a thickness of 1,000 feet, and thinning 

 out towards Canada, it crosses the Niagara river, at 300 feet in thickness, whence 

 it is traced Northwestwardly to Lake Huron, and thence to Mackinac. It is also 

 exposed in Pennsylvania, where it was called the Surgent red marl, and in West- 

 ern and Southern localities. In its lower part, it is made up chiefl}^ of marls and 

 thin shaly limestones, which include the gypsum and salt. Its upper portion 

 consists of magnesian limestones, often yielding hydraulic or water lime, and is 

 hence sometimes distinguished as the Water Lime Group, whele it really forms 

 part of the Onondaga Group. 



The Lower Helderberg Group has a wide geographical range, but is not sus- 

 ceptible of subdivision into many Groups, at any great distance from the Helder- 

 berg Mountains, where Vanuxem separated it into ; 1st, Water Lime Group or 

 Tentaculite limestone ; 2d, Pentamerus limestone ; 3d, Delthyris shaly lime- 

 stone ; 4th, Encrinal limestone ; and 5th, Upper Pentamerus limestone. The 

 Water Lime Group was so called from its yielding hydraulic cement, and is about 

 200 feet thick in New York, and thins out in Canada in a Northwesterly direc- 

 tion. Pentamerus limestone took its name from the Pentamerus galeatus found 

 in it, in Cherry Valley, New York, where it is about 30 feet thick. The Delthy- 

 ris shaly limestone was so named from the abundance of Spirifera macropleura^ 

 and 8. pachoptera^ formerly called Delthyris^ found in it. It is about 70 feet 

 thick. The Encrinal limestone is about 25 feet thick, and was so named from 

 the quantity of broken encrinites it contains. It has also been called the Scu- 

 tella limestone, from a shield-like pelvis of a crinoid found in it. The Upper 

 Pentamerus limestone is about 75 feet thick, and was so named from the abund- 

 ance of Pentamerus pseudogaleatus with which it is characterized. 



The Lower Helderberg attains the greatest thickness of 2,000 feet, at Gaspe, 

 Canada. It is 1,720 feet in Pennsylvania, 400 to 500 in New York, and from 100 

 to 200 feet thick in the Western States. 



The fossils of this Group are quite similar to those of the Niagara, and mark 

 a very gradual development from the former to the latter. The Water Lime 

 Group is especially characterized by large crustaceans of the genera Eurypterus 

 and Pterygotus, the highest forms of organized life, which, so far as we know, 

 had, up to this period, appeared upon the earth. 



The rocks of the Upper Silurian formation, as shown by the preceding esti- 

 mates, are about 8,000 feet in thickness. They contain the fossil remains of 

 no vertebrate animal so far as yet known. Thej' show the uninterrupted course of 

 oceanic life from one Group of rocks to the next, and the gradual appearance of 

 higher organisms, and yet they are without land plants, save perhaps a species 

 of Psilophyton, and vertebrate animals, even of the lowest oceanic types. The 

 student of biology and the laws of evolution may pause here and reflect upon the 

 fact, that from the geological horizon of the Eozoon canadense, we have passed 

 upwards through nearly eighteen miles in thickness of oceanic deposits, which 

 represent many millions of years as we understand the laws of deposition, and 

 while the changes in the forms of life have been numerous and wonderful, and 



