464 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



occasional individual would become embedded in the strata. Among 

 Eurypterids only the Pterygotus-like Erettopterus has a bilobed telson. 

 A small species of this genus moving on the sandy river bottoms or the 

 beach might perhaps have produced such a structure, if it used its telson • 

 for regular propulsion. Again, the organism may be referable to myrio- 

 podus types or to Insecta. Whatever the type which caused these trails, 

 there can be no question of its wide distribution, for it is found along the 

 whole Appalachian front. Nor can its preference for the higher shore 

 zone, if not for river bottoms, be doubted, since these markings are never 

 found in immediate association with marine fossils and are commonly 

 preserved by a cevering of sand, which shows cross-bedding of the eolian 

 type and which probably represents accumulations of blown sand. 



Mud cracks are common features in the red shales of the Medina east 

 of Eochester, and indeed the beds here have much more the aspect of 

 having accumulated for the most part above water. They appear to rep- 

 resent the dry land continuation of the beds forming the shore and lit- 

 toral deposits west of Eochester. The Thorold sandstone terminates the 

 Medina series, but has a thickness of only 5 feet in this section. A 

 peculiar structure, described by Conrad as Dictyolites beckii, covers a 

 large portion of the Thorold quartzite in this section. This has the 

 aspect of being some form of fimbriated mud crack, though it may have 

 a wholly different significance not yet ascertained. The Medina sand- 

 stone has not been recognized east of Oneida County, New York, its last 

 representation being in Cherry Valley. The underlying Queenston does 

 not extend so far, being unknown in the northern section east of Eome, 

 New York. The Thorold quartzite, however, extends farther east, be- 

 coming the Oneida conglomerate, which in the vicinity of Utica rests on 

 the eroded surface of the Frankfort shale. 



The red Medina beds are well exposed at Fulton on the Oswego Eiver, 

 where they are capped by hard, red sandstones carrying Arthrophycus 

 harlani. The succeeding Oneida conglomerate is here buried by the drift. 

 Eolian cross-bedding is common in these rocks, and the total thickness of 

 the series here is less than 100 feet. The contact with the Queenston is 

 not exposed and probably would not be recognizable. The base of the 

 section is formed by the Oswego sandstone, which causes a second fall 

 within the city of that name. 



In Oneida County the Oneida conglomerate alone occurs. South of 

 Utica this is about 40 feet thick and a pure quartz pebble rock, with 

 quartz sand as a matrix. The pebbles are well worn and indicate a sec- 

 ondary origin — that is, derivation from an older conglomerate — not only, 



