J 68 PHYSICAL GEOGEAPHY. 



to the triassic system ; but its exact geological position is 

 not yet satisfactorily determined. Cretaceous beds, known 

 by their organic remains (but not containing white chalk 

 with flints), occur extensively along the Missouri, and 

 spread widely on both sides of the Mississippi, below the 

 influx of the Ohio. They exist also near the Rocky 

 Mountains, high above the sea, in the vicinity of the 

 sources of the Arkansas, Platte, and Gila*; while towards 



21. Marcellus shales ; black, slaty, bituminous shales, containing 

 septaria, with occasional thin bands of limestone. Similar in 

 lithological characters to Genesee slate. 



22. Hamilton group ; an immense deposit of dull-olive calcareous 

 shales, which change to light ash-grey in weathering. It con- 

 tains septaria. 



23. Tully limestone ; usually thick-bedded, blue, or nearly black, 

 limestone, often divided by seams into irregular fragments. 



24. Genesee slate ; is a great mass of argillaceous, black, fissile 

 slate, which rapidly exfoliates and falls down. Fluid bitumen is 

 of common occurrence. Either this deposit or No. 21. exists on 

 the north side of Methy Portage, on the Elk or Athabasca river. 



25. Portage group ; avast deposit of shale, flag-stones, and thick- 

 bedded sandstone, rising with a slope or abruptly from the shales 

 on which it reposes. 



26. Chemung or Ithaca group ; a highly fossiliferous series of shales 

 and thin-bedded sandstones. 



To the last succeed the Old Red Sandstone and the Coal Mea- 

 sures. 



The lower part of the Champlain division has been considered as 

 the equivalent of the Cambrian system, the Utica slate being parallel 

 to the Landeilo flags ; the Hudson River group and Ontario division, 

 up to the Niagara group, is thought to be equivalent to the Caradoc 

 sandstone; the Niagara group and the whole Helderberg division is 

 supposed to be co-equal with the Wenlock rochs ; and the Erie 

 division as equivalent to the Upper and Lower Ludlow rochs, in- 

 cluding the Devonian system of Phillips. — Vide Hall, Geol. Report of 

 New York, p. 518. 



* " The cretaceous formations terminate in the Atlantic regions of 

 New York, before they have reached the city of New York, so that 

 their limit scarcely touches the 40th degree of latitude, or 16° lower 

 than in Europe. In Kentucky and Tennessee it remains below 37°, 



