l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6o 



Fruita, Colo., they have been correlated with the ..Morrison. The 

 lower part of the Gunnison near Grand Junction consists of flaggy 

 sandstones with a few layers of limestone, and rests unconform- 

 ably on " Red Beds" supposed to be of Carboniferous age. This 

 lower part of the Gunnison is doubtless equivalent to some part of 

 the La Plata group. Farther to the east, near Crested Butte, a white 

 sandstone near the base of the rocks there classed as ( iunnison prob- 

 ably represents the La Plata, for in the same general region Cross 

 and l.arsen,' the former of whom originally named and described the 

 La Plata, recognized it east of the town of Gunnison, where it over- 

 laps the older sedimentary rocks onto the Archean. 



The Dakota and Morrison formations are present in the inter- 

 montane basins, such as Middle I 'ark and South Park, although little 

 rs definitely known about their relations there. But east of the 

 mountains, at Morrison, Colo., is the type locality of the Morrison 

 formation. Between this formation and the underlying " Red P>eds " 

 (I ykins), formerly called "Upper Wyoming." there are beds of 

 sandstone and limestone which have been included in the Morrison, 

 but which are lithologieally different. Butler' has suggested that 

 these may represent the sandstones farther north (northeast Colo- 

 rado section), which he correlates with the Sundance. 1 am inclined 

 to regard them as the attenuated edge of the Nugget or lower 

 La Plata sandstone. It seems probable that the limestone and 

 gypsum at or near this horizon farther south 3 may represent the 

 extension of the Jurassic sea beyond the localities where its waters 

 were suitable for the support of marine organisms. 



NORTHERN ARIZONA TO SOUTHEAST COLORADO 



Relations still farther to the south are shown by sections situated 

 along a broken line extending from northern Arizona eastward to 

 Purgatoire Canyon in southeast Colorado. The Rico and Ouray sec- 

 tions are essentially the same as the La Plata section at the type 

 locality of the La Plata sandstone. A significant feature in this 

 southwestern area is the limestone and calcareous shale of the middle 

 of the La Plata group. Near Telluride,' situated between Rico and 



1 Cross, Whitman, ami l.arsen, E. S., Washington Acad. Sci. Jour, vol, 4, 

 1>. 237, 1914. 



-'Butler, G. M., Colorado Geol. Survey Bull. 8, 1014. 



'Richardson, G. B., U. S. Geol. Survey Geol. Atlas, Castle Rock folio 

 (No. 108), 1015. 



'Cross, Whitman. U. S. Geol. Survey Geol. Atlas, Telluride folio (No. 57), 

 1899. 



