470 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF XORTH AMERICA 



pebble conglomerate which, because of its origin from an older conglom- 

 erate and the thorough assorting of the material, became very pure. 

 Such is the Oneida conglomerate of Herkimer and Oneida counties, Xew 

 York, which rest disconformably on the eroded surface of the Frankfort. 

 The western extension of the white beds on the red Medina are seen in 

 the Thorold quartzite of Eochester, Xiagara, and the west, which has 

 been traced to Collingwood, on Georgian Bay, a distance of 300 miles 

 from the deposits in Pennsylvania. 



With the beginning of Medina transgression ensued again a period of 

 prolonged marine conditions, the sea extending farther and farther, to 

 be followed by a period of extensive retreat and laying bare of the conti- 

 nent in Salina time. 



TEE NIAGARAS MARINE INTERVAL 



The transgression of the Xiagaran Sea across the Xorth American 

 continent appears to have been of a rather complex type. It seems cer- 

 tain that there were at least two opposite movements, one from the north, 

 the other from the south. It appears that while Medina-Tuscarora sedi- 

 ments were still spread by the rivers from the Appalachian highlands, the 

 sea encroached from the region of the present Arctic Ocean and entered 

 the country by way of the upper Great Lake region. The Keppel dolo- 

 mites seem to have been the earliest deposits here, though farther north 

 still earlier ones may appear. From the south a transgressing sea entered 

 the Mississippi Valley, with the result that the Alexandrian formation 

 was deposited there. This formation contains a commingled Ordovicic 

 fauna, such as characterized the transition beds in the Atlantic Ocean, as 

 shown by the Anticosti section. When a junction was effected between 

 the northern and southern encroaching seas, a certain commingling of 

 the faunas took place. On the whole, however, the faunas remained more 

 or less distinct, as shown by a comparison of fossils from the Wisconsin 

 and Kentucky -Tennessee region. TJlrich misinterprets this difference of 

 faunas as due to variation in horizon. This leaves out of consideration 

 the common facts of modern fauna! distribution.* The hiatus which 

 Ulrich places between the Eochester and Lockport, in order to make room 

 for some southern formations, is absolutely non-existent. I have for 

 years studied these formations in the Xiagara Gorge, and I am absolutely 

 convinced that no such hiatus exists. The Eochester shale by becoming 

 more calcareous passes without a break upward into the Lockport dolo- 

 mite. To be sure, there is a peculiar type of deformation in the basal 



* For a summary of these, see Principles of Stratigraphy, chapters xxvi to xxix. 



