The Position of the Cincinnati Qroup. 107 



crease of calcareous matter. The upper or terminating limestone of 

 the Clinton Group is succeeded by a soft argillo-calcareous shale, which 

 maintains its character unchanged for a thickness of 80 to 100 feet. 

 Thi'oughout the greater part of this it abounds in fossils, nearly all of 

 Avhich are quite distinct from those in the beds of the Clinton Group. 



" In the western part of New York, the lithological characters of 

 the Clinton and Niagara Groups are so similar that they could well be 

 united. The fossils also of the two groups, though generally distinct, 

 are nevertheless generically similar, and several species pass from the 

 lower to the higher group. The upper limestone of the Clinton Group, 

 which forms a strong line of demarkation between the two, contains in 

 its western extension several fossils usually regarded as peculiar to the 

 Niagara Group. Among these may be noticed the Caryocrimis ornatus, 

 and the Hypanthocriniis deeorus. It can not be denied, therefore, that 

 there is a gradual approximation in the two groups, both in lithologi 

 cal and paleontological characters, as we trace them westward within 

 the limits of the State of New York. Still farther west, the assimilation 

 becomes more perfect, and there appears to be no line of separation 

 between the two groups. At the same time the fossils appear to be 

 commingled." (Pal. of N. Y., vol. iii., p. 106.) 



The Onandaga Salt Group takes its name from Onandaga county, 

 New York, where the salt springs have been extensively wrought. It 

 is sometimes called the Onandaga limestone, or the Gypsiferous series. 



Its outcrop in New York is traced from Montgomery county, where 

 the formation is represented by a thin band, westward, till it attains, in 

 Wayne county, a thickness of 700, and even 1,000 feet, and, finally, re- 

 duced once more to less than 300 feet, crosses the Niagara river into 

 Canada, whence it is traced northwestward, forming a band between 

 the outcrops of the Corniferous and Guelph formations to Lake Hu- 

 ron, and thence to Mackinac. (Geo. of Can., 1866, p. 263.) 



This formation, in its lower part, is made up chiefly 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 

 gi'oup, though really forming a part of the Onandaga formation. It 

 attains a thickness in Canada of 800 to 1,000 feet. (Geo. of Can., 

 1866, p. 237.) But in Pennsylvania, where it is called the Surgent 

 red marl, it is only 350 feet thick. • 



The Lower Helderberg Group is so termed from its complete develop- 

 ment along the base of the Helderberg Mountains. In some parts of 

 the Helderberg Mountains, and in other parts of New York, it rests di- 

 rectly upon the water lime beds, which is the uppermost member of the 

 Onandaga salt group. 



