SILURIC SECTION, ROCHESTER TO LAKE HURON 305 



wise the transition between the Clinton and Rochester is quick, but 

 no break in sedimentation is apparent. Then, too, many of the species 

 are common to both formations. Below there is a gradual transition 

 into the 



Williamson shale member, 24 feet thick. Thins rapidly to the west and 

 is absent at Lockport. A green shale series at the top (8 feet), below 

 which are more green shales (4 feet) with very thin pearly lime- 

 stones replete with Coelospira hemispherica, and finally purplish and 

 black shales (12 feet), the latter with Monograptus clintonensis and 

 RetioUtes venosus in abundance. 



Wolcott limestone member, 14 feet thick. Thin-bedded greenish lime- 

 stones that come in rather sharply over the Sodus shale, but shade 

 into the Williamson. Three feet above the base occurs the Furnace- 

 ville iron-ore, here about 1 foot thick. It is decidedly cross-bedded, 

 made up of fragmented and worn fossils, some sand, and less oolite ; 

 the fossils are crinoidal fragments, Bryozoa, and Tentaculites, all 

 altered or coated with iron. The beds below the ore and the ore 

 itself show wave action. Pentamerus oblongus (Clinton form) and 

 Hyattidina congest a are guide fossils. 



Sodus shale member, 24 feet thick. A green, fine-grained shale almost 

 devoid of fossils, other than "fucoids" and trailings. It rests abruptly 

 and without the least transition upon the Medina. The Sodus thins 

 rapidly to the west and is only 3 feet thick at Medina. 

 Contact very sharp between adjacent beds. Probably no time break. 

 Medina formation. Thickness about 60 feet. 



Thorold member, or "gray band." A massive white sandstone, 5 feet 

 thick. 



Heavy-bedded, channeled, red and mottled, dirty sandstones, with but 

 little shale and many zones of intraformational shale pebble con- 

 glomerates, 15 feet thick, with Dwdalus archimedes throughout and 

 Arthrophycus alleghaniense in upper 6 feet. 



Regularly thin-bedded, lighter red sandstones, with more prominent shale 

 partings, about 20 feet thick. A. alleghaniense and Lingula cuneata 

 in upper 5 feet and much sun-cracking in upper 2 feet. 



Basal thick-bedded, very coarse red sandstones, as follows: At the top 10 

 feet of irregularly bedded sandstones, followed by one bed 4 feet 

 thick, also much cross-bedded, and then the basal zone of 6 feet, regu- 

 larly bedded below, and cross-bedded and often deeply channeled 

 above. Here again the sandstone fills into the sun-cracked surface of 

 the Queenston below. 

 Disconformity. Base of Siluric. 



Queenston formation. Top of Ordovicic (Richmondian). There has been 

 much uncertainty here as to the lower limit of the Medina, but the 

 heavy, and much cross-bedded gray sandstones easily mark the base. 

 The uncertainty is due to the fact that the Queenston is here much 

 more sandy than farther west and that there are horizons of local 

 sandstones. Still farther east these sandstones pass into the Oswego. 



Here the Queenston in the upper 40 feet consists of red, micaceous, sandy 

 shales, with thin zones of gray localized sandstone. There are many 



