STRA TI GRAPHIC SUM MAR Y. 62 1 



on the Ordovicic or Upper Cambric. At Canyon City, Colorado, 

 the Harding sandstone, with a lower Trenton fauna, rests upon 

 the crystallines and is succeeded by the Fremont limestone with an 

 Upper Trenton fauna. In the Bighorn Mountains, the Bighorn 

 limestone of Black River — early Trenton age rests by overlap on 

 the Upper Cambric, and is succeeded disconformably, after a great 

 hiatus, by the Madiso^n limestone of Mississippic age. In the 

 northern portion of the Bighorn uplift, shaly beds with an Upper 

 Richmond fauna succeed the Bighorn limestone, being separated 

 from it by a considerable hiatus and by an even greater one from 

 the overlying Mississippic limestone. In the Black Hills, the 

 Whitewood limestone with an Upper Trenton fauna rests upon 

 the Upper Cambric Deadwood and is succeeded by Mississippic. 



West of the Front Range in Colorado, the Orodovicic Yule 

 limestone succeeds the Upper Cambric. It probably represents 

 the Beekmantownian division. 



In the Wasatch Mountains, the Ute limestone represents Upper 

 Cambric and Lower Ordovicic. It is succeeded by a quartzite 

 formerly called Ogden and believed to be in the main, a continental 

 formation; and this in turn is followed by Siluric and Devonic 

 strata. In Central Nevada, the Pogonip limestone ranges in age 

 from Upper Cambric to Lower Ordovicic. Its upper portion is 

 probably Beekmantownian, with a fauna largely distinct from the 

 corresponding eastern faunas, owing to a general geographic sepa- 

 ration. It is separated by the Eureka quartzite, an extension of 

 the " Ogden," from the Lone Mountain limestone of Upper 

 Ordovicic (Trenton-Cincinnati) age. 



In the Yellowstone region, the Jefferson limestone represents the 

 Lower Ordovicic. It rests upon the Gallatin limestone of Upper 

 Cambric age and is succeeded after a hiatus by Devonic sediments. 



In Alaska, some graptolite bearing beds and metamorphic lime- 

 stones, schists and quartzites are referred to the Ordovicic. 



III. The Siluric System. 

 General Subdivision. — The Siluric of North America admits of 

 the following subdivisions :^^ 



" Grabau, loc. cit. 



