DEVONIAN. i50y 



character the cleavage is irregular and oblique to the planes of stratification. On weathering, 

 where the edges are exposed, there is manifested a slight tendency to slaty cleavage. 



From the wide and even distribution of the materials of this group, it was evidently pro- 

 duced during a period of great tranquillity, when the finely levigated mud was transported 

 over wide areas by gentle oceanic currents. The great profusion and variety of organic forms 

 proves also the quiescent condition of the ocean, which, together with the slightly calcareous 

 nature of the mud, favored the growth and distribution of the Testacea. 



The upper part of this group, in the Fourth district, bears a very close analogy in its litho- 

 logical nature to the shale of the Niagara group, and in abundance of organic remains it is even 

 more prolific. The forms of the latter are, however, of entirely different species, though corals 

 and shells of similar genera abound, and trilobites of the same and different genera. 



Concretions or septaria, in well-defined and often fantastic forms, are common in every part 

 of this group. In many instances the calcareous matter has concreted around some organic 

 body, or a nodule of iron pyrites seems to have been the center of attraction. In such instances 

 we often find numerous fossils embedded in them or attached to their outer surface. The 

 greater number, however, are well-defined spheroidal masses, with or without seams of crystal- 

 line matter and not containing any organic body. Others, and particularly in the lower part 

 of the shale, are small, spherical or elongated, and with a small perforation through their center, 

 in the manner of the common nodules or other concretionary forms in recent clay beds. 

 ' Organic forms abound throughout the group, but they vary somewhat in the different 

 parts. In the lower division the most abundant are those of Orthis, Atrypa, and Strophomena, 

 with some spiral univalves; while above this portion great numbers of Avicula, Cypricardia, 

 Nucula, and other similar forms abound, with fewer of the genera Orthis, Delthyris, etc. In the 

 next division Delthyris, Strophomena, and Atrypa abound, to the almost entire exclusion of 

 the forms before mentioned. In the same situation with these we find numerous species of 

 corals; CyathophyUi, Favosites, and other forms are abundant, while fragments of crinoidal 

 columns are everywhere scattered through the mass or, spread evenly over the surface, form 

 thin layers of themselves. Many of the species of this division are discontinued and their 

 place supplied by o'thers of the same genera in the higher part of the group. * * * 



Although this group is so widely spread and evenly distributed and of uniform character 

 over the western part of the State, still at its eastern extremity the lithological character is 

 widely different. The shales are more or less arenaceous and some parts are weU-marked 

 sandstone. The proportion of siliceous and argillaceous earth is nearly reversed from what it 

 is in the same rocks farther 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 increasiag. 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 southeast. The force of the current which drifted them iato the ocean was sufficient only 

 to carry onward the coarser particles to a certain distance, where they were deposited. The 

 finely levigated mud was carried beyond this point, being floated by less force than the sand. 

 Some portion of the clay was deposited with the sand toward the central part of the State, and 

 but little of the latter extended beyond this point. Finally the current became more gentle 

 and the clay was deposited to a certain extent, beyond which the power of the current was 

 iasufficient to carry even this material, and the deposit consequently thinned out in that 

 direction. * * * 



This change in the nature of the materials is accompanied by an equally marked change 

 in. the fossils embedded in the different parts of the group. In the eastern part of the State 

 Avicula and Cypricardia, with Nucula, etc., prevail in immense numbers, while at the extreme 

 western margin, though in precisely the same position, they are of the rarest occurrence, whUe 

 numerous forms of Delthyris and Atrypa abound. 



The above-quoted descriptions by Hall apply particularly to the Fourth dis- 

 trict of the early Geological Survey of the State — that is. the region south of Lake 



