204 GEOLOGY OF THE DENVER BASIN. 



formation proves the great extension of andesitic lavas on the western 

 slope of the Colorado Range at the time assumed for the lava floods on 

 the eastern side, and makes the assumption as to their magnitude far less 

 venturesome than if it were unsupported. 



The other discoveries which support the views advanced as to the 

 great andesitic eruptions adjacent to the Denver sea are the identification 

 of probably contemporaneous lake deposits, also composed of andesitic 

 debris in great degree, in South Park, near Canyon, on the Animas River 

 below Durango, and in the Elk Mountains, all in Colorado. These forma- 

 tions, which will be referred to more in detail in discussing the age of 

 the Arapahoe and Denver formations, testify to very extensive contempo- 

 raneous andesite outbursts in various parts of Colorado, and make the 

 conclusions reached for the Denver area seem quite probable. 



In his most valuable presidential address before the Colorado Scientific 

 Society, "Structural and Orographic Features of Rocky Mountain Geol- 

 ogy," Mr. li < !. Hills 1 has argued that the andesitic materials of the Denver 

 beds may more plausibly be derived from the known andesitic eruptions of 

 South Park, through the drainage of the South Platte River, than from lava 

 masses adjacent to the Denver sea. This hypothesis seems to the writer 

 entirely untenable in view of the fact that the lower sediments of the Denver 

 sea were accumulated very slowly, as shown by the character of the fine- 

 grained clays, sands, and gravels of the section at the northeast extremity 

 of South Table Mountain. These sediments must have contained a large 

 proportion of material from the adjoining Archean continent had those 

 materials been exposed during their deposition. Furthermore, the study 

 of the Pikes Peak district" has made it appear probable that during the 

 Denver epoch the drainage of South Park was to the southeast, into the 

 Canvon bay, ami that the Eocene volcanic accumulations to the south of 

 the park diverted the chief drainage channel of the park into the present 

 course of the South Platte River. 



In connection with the theory of vast lava floods of andesite covering 

 the slope of the continental area adjoining the Denver sea on the west, it 



iProc.Colo. Sci. Soc, Vol. Ill, Part III, 1891, pp. 359-458. 



^Geologic Atlas of the United States, folio 7, Pikes Peak, U. S. Geol. Surv., 1894. Geologj by 

 Whitman i IrOBS. 



