746 HISTORICAL GEOLOGY. 



often called brownstone. The material of many of the "brownstone fronts" of New 

 York and other eastern cities is mostly from this formation. The Potomac conglomerate 

 marble is used as an ornamental stone, and columns of it stand in the Capitol at 

 "Washington, 



2. The Triassic and Jurassic of the "Western Interior and Pacific Border Regions. 



The Triassic and Jurassic formations of the Western Interior and of the 

 Pacific border have a wide distribution, and, to some extent, distinguishable 

 limits. The former consist almost everywhere, in the Interior, of reddish 

 sandstones and marlytes, and are often called '• Ked Beds." They frequently 

 contain gypsum and sometimes salt. Upon the Pacific border the rocks of 

 this period are chiefly slates, with occasional sandstones, and much limestone. 



The Jurassic beds are usually of lighter shades of color, and are in most 

 regions partly or chiefly calcareous, and the limestone is often cherty. A 

 large part of the Triassic formation is without fossils, excepting occasional 

 traces of plants ; but the Jurassic is often fossiliferous, though seldom 

 prolific in species. 



Triassic. 



Over the Continental Interior, the Triassic formation is exposed to view 

 in northern Texas, adjoining Indian Territory and western Kansas. The 

 beds probably underlie the Cretaceous beds farther northward, but no out- 

 crops occur in that direction except in mountainous regions to the west and 

 northwest. They exist about the Black Hills of Dakota, and cover large 

 areas along the Summit Region of the Eocky Mountains in New Mexico, 

 Colorado, and Utah, east of the western limit of the Wasatch Eange, and 

 also in Wyoming, Montana, and Idaho. In British America, east of the 

 Archsean protaxis, they have been observed on Peace and Pine rivers, 

 beyond 55° N. and between 122° and 125|-° W. ; and also on Liard Eiver, 

 near 59° N. Beds in southeastern Idaho, near Soda Springs, have been 

 referred to the Lower Trias (Mojsisovics, Hyatt) ; but the absence ar non- 

 discovery of fossils leaves the age of the beds of the Eocky Mountains and 

 Interior Continental regions generally undetermined. 



West of the meridian of the Wasatch Mountains, and of the Eocky Moun- 

 tain protaxis in British America, over the Great Basin plateau, and its con- 

 tinuation in the plateau region of British Columbia, the Trias appears to have 

 a wide range. In the United States it is confined to the west side of the 

 plateau or Great Basin beyond 117|-° W., on the 40th parallel. In the Avest 

 Humboldt region, according to King, 15,000 feet of beds, partly Middle Trias, 

 underlie 4,000 feet or more of Jurassic beds. In the plateau region of 

 British Columbia, Triassic areas occur on Nicola Lake (50° N., 120^-° W.) 

 and Stikine Eiver (57° N., 137i° W.). 



Farther west, in the Sierra belt, beds of the Upper Triassic occur near 

 the summit of the Sierra Nevada in Plumas County, Cal., as first identified 



