SILURIC SECTION, ROCHESTER TO LAKE HURON 309 



Dalmanites Umulurus, Homalonotus delphinocephalus, Lichas doltoni, 

 etc. 

 Clinton formation. Thickness about 30 feet. 



Irondequoit limestone member, 10 to 15 feet thick. Very heavy-bedded, 

 crystalline, crinoidal, pinkish gray limestone, with occasional Bryozoa 

 reefs within and at the top of the limestone. In the latter case the 

 reefs project several feet into the Rochester. Also has zones of 

 stylolites. Transition into Rochester quick, almost abrupt. Fauna 

 essentially that of the Rochester shale. 



Wolcott limestone memler, 12 to 21 feet thick. Thin-bedded magnesian 

 limestones with a sparse fauna. Pentamerus ohlongus, Ccelospira 

 plicattila, Hyattidina congcsta. 



Basal shale, 2.5 to 6 feet thick. Green to grayish shales, with Ccelospira 



hemispherica and C. plicatula. Rests abruptly on the Medina. This 



zone is often correlated with the Sodus member at Rochester, but 



there is nothing of value to support this reference. 



DisGonformity. Contacts sharp between adjacent formations. Sodus shale 



absent. 

 Medina formation. Thickness about 65 feet. 



Thorold member. Massive, greenish white, cross-bedded sandstone, 8 feet 

 thick. The "gray band" of Eaton. 



Red and greenish gray, much cross-bedded and channeled sandstone, with 

 very little shale, about 15 feet thick. ArtJirophycus alleghaniense and 

 Lingula cuneata occur 2 feet beneath the top. 



Thin-bedded red sandstones, with considerable red shales, and two or 

 more zones of localized storm-rolled mud balls (concretions of 

 authors), 85 to 40 feet thick. 



Gray sandstone with green shale partings/ 5 feet thick. Poor Medina 

 fossils here, noted by Hall in 1838. 

 Cataract formation. Thickness about 54 feet. Seen best on each side of the 

 small tunnel and in Evan's gully, on N. Y. 0. R. R. 



Upper dark green shales, 5 feet. 



Thin-bedded green sandstone at top, followed by yellowish magnesian 

 and argillaceous limestone, with small black shale pebbles.- 5 feet, 

 abounding in Helopora fragilis, fragments of Lingula, Camdfotcechia 

 neglecta, Isochilina cylindrica, and .small gastropods. 



Middle green shales, 10 feet. 



Dark green shales, with very thin-bedded argillaceous magnesian lime- 

 stones, 5 feet. Helopora fragilis common, Lingula in fragments, 

 Camarotoechia neglecta, and Whitfieldella. 



Lower dark green fissile shales, 7 feet. 



Whirlpool sandstone member (Grabau, 1909), 22 feet thick. Heavy- 

 bedded, clean, white, somewhat coarse, cross-bedded sandstone. Thin- 

 bedded in upper 5 feet. No fossils seen in the gorge exposures. See 

 plate 13, figure 1. 

 Disconformity. Base of Siluric. The slightly undulatory contact with the 



Queenston is well shown along the Grand Gorge trolley line. 

 Queenston (Grabau). Top of Ordovicic (Richmondian). Exposed for 115 

 feet. Thickness in deep wells 1,085 feet. Brick-red sandy shales 

 with oxidized green streaks. No fossils. 

 XXII — Bull. Geol. Soc. Am., Vol. 25, 1913 



