Belated Formations. 



11 



and is equally divided between the Trenton and Nashville series. 

 This series is a natural group^ and though each member has many 

 of its own, yet there are a number of forms uniting the two. It is 

 divided both on lithological and palaeontological grounds into two 

 sub-groups. The Trenton ends with a light colored, heavy-bedded 

 limestone, and the Xashville begins with a silicious, blue calcareous 

 rock, weathering often, into thin, earthy, buff, sandy masses, and, 

 sometimes, into shales. The bed is distinguished by the abundance 

 of Orthis testudinaria and Strophonema alternata, while the upper 

 Trenton beds contain Stromatopora rugosa, Columnaria alveolata, 

 Petraria profunda^ Strophonema filitexta^ Rhynchonella recur- 

 virostra^ Pleurotomarialapicida, Orthoceras Huronense^ etc. 



"The Orthis bed is frequently a group of smoothly laminated 

 flags, interstratified with shaly seams, when wet it looks much like 

 the 'Black Shale.' At almost all points where it comes to the surface 

 throughout middle Tennessee, it is seen to contain vast numbers of 

 individuals of Orthis testudinaria. 



" In the western valley of Tennessee it is the hydraulic rock and 

 the most conspicuous of the Nashville strata. Along the Tennessee 

 river it is seen at the base of several bluffs, its dark band on a 

 level with the water and in strong contrast with the lighter beds, 

 and gray limestones of the high bluffs above. 



" The upper members of the Xashville group, constitute a group 

 of rather dark blue, highly fossiliferous, often roughly bedded, im- 

 pure, limestones of a maximum thickness of 400 feet." 



The preceding descriptions, selected from the various authors 

 who have published details of the character and sequence of the 

 strata of the area and geological horizon over which our examina- 

 tions have taken us, proving the existence of a widespread change 

 in the physical condition of the sea of the Appalachian basin at the 

 close of the Trenton limestone formation, and preceding that of the 

 coarser and more varied sediments of the Hudson River formation, 

 are given as a basis for including the Utica shale, Galena limestone, 

 Triarthrus beds of Cincinnati, and the Orthis bed of the Xashville 

 series, in one geological epoch; we might also add the Thebes sand- 

 stone and a portion of the Graptolitic shales of Virginia, Tennessee 

 and Alabama. 



All along the margins of the great Appalachian basin, from Anti- 

 costi to Lake Huron, to Minnesota, to Alabama and thence north- 

 east to the St. Lawrence, the change at the close of the Trenton 

 limestone-forming period, in the lithological characters and the 

 fauna, is everywhere apparent; in some localities quite maiked and 

 abrupt; in others the horizon is nearly lost by the blending of both 

 the strata and organic remains of the lower with the next succeeding 

 formation. The organic remains of the Utica slate entitle it to rank 

 as a formation quite as much as its lithological character. one 



