380 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Vanuxem found smooth arborescent fucoids in this group. 



Fossils occurrent in Europe. — There is only one, Productus sub- 

 aculeatus (De Vern.). It is both in Russian and Rhenish Devonian. 



Fossils recurrent in New York. — Almost its only relations (and 

 they are npt close) are with the Genesee Slate, to which, though not 

 near in place, its mineral composition is very similar ; Tentaculites 

 Jissurella, Leptcena (Stroph.) setigera, and Avicula fragilis are the 

 only species common to the two groups. 



The highly fossiliferous group (the Hamilton) has only one Mar- 

 cellus fossil, the Homalonotus Dekayi ; and according to Hall it is 

 rare. 



Hamilton Group*. 



Mineral Character. — This group may be called a great develop- 

 ment of dull-olive or bluish-grey calcareous shales, weathering grey 

 or brownish. It consists of shale and sandstone in endless mixture, 

 in three distinct mineral masses as to kind, the sandy portions 

 being generally in the middle (Vanuxem). Concretions or septaria 

 are common in every part of this group, in well-defined forms 

 gathered round a nodule of iron-pyrites or some organic body (Hall). 



The three portions into which this group has been rightly divided 

 are, beginning from above, — 1st, the Moscow Shales ; 2nd, Encrinal 

 limestone ; and 3rd, the Ludlowville Shales (often called Skeneateles 

 and Olive Shales). 



The Moscow Shales are dark-blue, finer and more calcareous than 

 the others, and very like the Niagara Shales of Central New York. 

 They terminate upwards in the Tully Limestone, soon to be men- 

 tioned ; and they contain septaria. 



The Encrinal Limestone is impure, tough, brown, and full of 

 Echinodermata. It is persistent and always found on the same 

 horizon. 



The Ludlowville Shales differ from the above in being sandy. 



James Hall (Report, p. 165) makes the following useful remarks 

 on this group : — " Although this group is so widely and evenly dis- 

 tributed, and of uniform character over the western part of the State, 

 still at its eastern extremity the lithological character is widely dif- 

 ferent. The shales are more or less arenaceous ; and some parts are 

 well-marked sandstone. The proportion of siliceous and argillaceous 

 earth is nearly reversed from what it is in the same rocks further 

 west. The mass varies from sandy shale to shaly sandstone, and 

 even tolerably pure sandstone. This character gradually changes to 

 the westward, — the sand diminishing, and the clay increasing. 



"The features presented by this group at its two extremes and 

 along its whole length offer one of the most instructive exhibitions 

 of the varying character of mechanical deposits. The facts prove the 

 origin of the materials to have been at the east or south-east. 



" The force of the current which drifted them into the ocean was 



* Professor H. D. Rogers characterizes this group, as he met with it in Penn- 

 sylvania, as a bluish-grey, brownish, and olive-coloured sandstone, sometimes cal- 

 careous (Atlas, Johnston). 



