SISVER AND GOLD. 205 



strata in considerable thickness, but to the west they dip under 

 the vastly greater development of Mesozoic terranes which shade 

 out into the Colorado Plateau. In northern, central, and south- 

 western Colorado are vast developments of igneous rocks that 

 have attended the geological disturbances. 1 



2.09.10. The San Juan region includes several counties in 

 southwestern Colorado, in whole or in part, viz.: Ouray, Hinsdale, 

 San Juan, Dolores, and La Plata. The chain of the San Juan 

 Mountains consists of great successive outflows of eruptive rocks, 

 porphyry, diabase, diorite, basalt, etc., which cover up the Archfe- 

 an and later sedimentary terranes, except in a few scattered ex- 



1 G. L. Cannon, " Quaternary of the Denver Basin/' Proc. Colo. Sci. 

 Soc., III. 48. See also III. 200. W. Cross, "The Denver Tertiary Forma- 

 tion," Amer. Jour. Sci., iii., XXXVII. 261. G. H. Eldredge, "On the 

 Country about Denver, Colo.," Proc. Colo. Sci. Soc., III. 86. See also 140. 

 S. F. Emmons, " Orographic Movements in the Rocky Mountains," Geol. 

 Soc. of America, I. 245-286. F. M. Endlich, "On the Eruptive Rocks of 

 Colorado," Tenth Ann. Rep., Hayderis Survey. H. Gannett, " Report on 

 the Arable and Pasture Lands of Colorado," Hayden's Survey, 1876, p. 313. 

 H. C. Freeman, " The La Platte Mountains," M. E., XIII. 681. G. K. Gil- 

 bert, " Colorado Plateau Province as a Field for Geological Study," Amer. 

 Jour. Sci., iii., XII. 16,85. J. D. Hague, Fortieth Parallel Survey, Vol. 

 III., p. 475. F. V. Hayden, Reports of Hay den's Survey, 1873, 1874, p. 40 ; 

 1875, p. 33 ; 1876, pp. 5, 70. R. C. Hills, "Preliminary Notes on the Erup- 

 tions of the Spanish Peaks," Proc. Colo. Sci. Soc., III. 24, p. 224. "The 

 Recently Discovered Tertiary Beds of the Huerfano River Basin," Proc. 

 Colo. Sci. Soc., III., pp. 148, 217. " Jura-Trias of Southeastern Colorado," 

 Amer. Jour. Sci., iii., XXIII., p. 243. A. Lakes, "Extinct Volcanoes in 

 Colorado," Amer. Geol., January, 1890, p. 38. Oscar Loew, " Report on the 

 Minerals of Colorado and New Mexico," Wheeler's Survey, 1875, p. 97. 

 " Eruptive Rocks of Colorado," Wheeler's Survey, 1873. C. A. H. McCauley, 

 "On the San Juan Region," Rep. Chief of U. S. Engineers, 1878, III., p. 

 1753. C. S. Palmer, "On the Eruptive Rocks of Boulder County," He., 

 Proc. Colo. Sci. Soc., III., p. 230. A. C. Peale, " On the Age of the 

 Rocky Mountains in Colorado," Amer. Jour. Sci., iii , XIII., p. 172 ; Reply 

 to the above by J. J. Stevenson, Amer. Jour. Sci., iii, XIII. 297. S. H. 

 Scudder, "The Tertiary Lake Basin at Florissant," Hayden's Survey, 1878, 

 p. 271 ; see also 1877. J. A. Smith, Catalogue of the Principal Minerals 

 of Colorado, Central City, 1870. J. J. Stevenson, "Notes on the Laramie 

 Group of Southern Colorado,'' Amer. Jour. Sci., iii., XVIII. 129. "The 

 Mesozoic Rocks of Southern Colorado," Amer. Geol, III., p. 391. P. H. 

 VanDiest, "Colorado Volcanic Cones," Proc. Colo. Sci. Soc., III., p. 19. 

 C. A. White, " On Northwestern Colorado," Ninth Ann. Rep. Director 

 U. S. Geol. Survey, 683-710. 



