NIAGARA PERIOD — MEDINA EPOCH. 231 



Fucoides {Arihrophycus) Harlani, fig. 358, — a very common fossil in the 

 Medina sandstone. It suffices to prove that the Oneida epoch is 

 more closely related to that following than to the preceding, and 

 hence belongs to the Upper Silurian era. 



2. MEDINA EPOCH (5 b). 

 I. Rocks : kinds and distribution. 



In the Interior Continental basin the Medina formation is not known, 

 excepting to the northeast. It occurs in western New York, over- 

 lying the Oneida conglomerate ; towards Utica it thins out, and is 

 not known east of there. On the Niagara the beds have a thick- 

 ness of 350 to 400 feet. It spreads northwest, and has been recog- 

 nized as far as the Straits of Mackinac. 



Along the Appalachian region it extends south through Penn- 

 sylvania and Virginia, where in some places the beds are over 1500 

 feet thick. 



The rocks are argillaceous sandstones, and marls of red, gray, and 

 mottled colors. The sandstones are thinly laminated, as is usual 

 when much argillaceous, and in general are quite fragile. 



In the Eastern border region, at the island of Anticosti, just north of 

 Nova Scotia, limestones follow those referred to the Hudson period, 

 without interruption ; the strata, according to the description in 

 Logan's Canadian Keport, have a thickness — reckoning from the top 

 of the beds of the Hudson period, up to those which are regarded of 

 the Clinton epoch which follows the Medina — of 700 feet ; they are 

 said to contain about thirty species of fossils common to the Lower 

 and Upper Silurian, and are described as beds of passage between 

 the two eras ; but according to Shaler, all the beds of the lime- 

 stone formation are Upper Silurian. 



The relation of the Medina group to Fig. 357. 



the overlying Clinton and Niagara A,,—-, 



groups is well illustrated in one or two §& ; -JllIB? 



sections from the western part of the : j^ ""' ^ ^S^ =^M 



State of New York. Fig. 357 repre- $ c I ^!~g!g|j | ||OZ!3 4 



sents the rocks at Genesee Falls, near \ ygj •'T^ ^^^^^^T' 7 ??^ ^ 



Rochester. The lower strata, 1, 2, are ■ _ ^dZ^~~ ---S^ l 



the Medina sandstone (5 b); 3, 4, 5, 6, Section at Genesee Falls. 



the Clinton group (5 c); and 7, 8, the 



Niagara group (5 d), — 2 heing a grit rock, 3 and 5 shales, 4 and 6 limestone, 7 



shale, and 8 limestone. The whole height is ahout 400 feet. 



The following figure (357 A) represents a section of the rocks along Niagara 

 Biver, from the bluff at Lewiston (L) to the Falls at F, passing by the Whirlpool, 

 at W, — a distance of seven miles. 



