156 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



23. Arkose sandstone, cross-bedded and conglomeratic in part; this layer is purple at the base, and 

 this color alternates with greenish-yellow and red bands; in the upper part a white and cream 



color prevails 30 



22. Sandstone, rather flaggy, containing a layer of gnarly limestone at the base 18 



21. Calcareous sandstone, shaly in the lower part and containing gnarly limestone near the top; color, 



bright red 10 



20. Sandstone, micaceous, massive, becoming flaggy at the top, where it contains limestone nodules; 



color, red , 5 



19. Calcareous sandstone, containing small limestone nodules near the top; color, red 5 



18. Calcareous sandstone and red arkose, containing limestone pebbles; purple and white in the 



upper part; red below 12 



17. Covered 12 



16. Arkose sandstone, variegated, red, white, and purple 6 



15. Sandstone, micaceous, flaggy 5 



14. Arkose sandstone, flaggy, becoming finer grained and micaceous in the upper part; color, pink, with 



narrow white bands 15 



13. Arkose sandstone, somewhat conglomeratic in the upper part, and with thin shaly bands near the 



center and at the top; color, white and pink, irregular bands 35 



12. Sandstone, micaceous, flaggy, very thin bedded at top; color, red 10 



n. Calcareous sandstone; a few thin layers of nodular limestone; color, red 7 



10. Sandstone, micaceous, flaggy; color, red 6 



9. Sandstone, micaceous, flaggy, becoming more compact in upper part 15 



8. Arkose sandstone, white, banded with red 7 



7. Calcareous sandstone, rather poorly exposed in the upper part, but flaggy and somewhat shaly, 



becoming more shaly in the lower part, and containing nodules of limestone; color, red 35 



6. Arkose sandstone, red above, white below 6 



5. Shale, probably calcareous; contains nodules of limestone; color, red 5 



4. Sandstone, micaceous; color, dark purplish red 5 



3. Calcareous sandstone, irregularly nodular and flaggy; contains gray nodules of limestone, but no 



well-defined limestone ; color, red 25 



2. Arkose sandstone, micaceous, thin conglomeratic, cross-bedded; near the base there is a thin 



black shale which is quite variable; color, white, with bluish and green zones 15 



i. Calcareous sandstone, with a gnarly gray limestone at the top; color, red 10 



Total 1,508 



The most complete section of the Cutler occurs in the Dolores Valley, 

 below Rico, Colorado. 1 There is exposed 98 feet of the formation, the 

 upper 32 to 35 feet are arkosic and the rest shale and sandstone with a 

 subordinate amount of limestone; a section is given on page 48 of the 

 article cited. The space immediately below the Cutler is covered and it is 

 not known whether the Rico is present or not. 



The Hermosa is made up of 490 feet of shale, sandstone, limestone, and 

 grit. (Section on page 48 of Cross and Larsen.) 



The Molas is at least 100 feet thick, mostly calcareous shale. There is 

 much of the formation that is "almost a red clay, but where the calcareous 

 element is prominent the mass becomes irregularly nodular or lumpy." 

 Layers or lenses of limestone are rare. This formation is persistent through- 

 out the San Juan region. 



In 1910, Cross and Spencer 2 first described the Rico formation: 



"It is here proposed to apply the name Rico to a formation assumed to be 

 about 300 feet in thickness, occurring between the Hermosa or characteristic 

 Pennsylvanian Carboniferous and strata assigned at present to the Trias of the 



1 Cross, Whitman, and E. S. Larsen, Contributions to the Stratigraphy of Southwestern 

 Colorado, U. S. Geological Survey, Professional Paper No. 90, p. 39, 1914. 



1 Cross, Whitman, and A. C. Spencer, Geology of the Rico Mountains, Colorado, 2ist 

 Annual Report U. S. Geological Survey, part n, p. 59, 1900. 



