AGE OF THE ARAPAHOE AND DENVER. 245 



The only question is as to the extent of the separation from the Laramie, 

 and more knowledge in various directions is necessary to determine this 

 point h is noticeable that the developments of the past few years have 

 steadily increased the importance' of these formations. The evidence of 

 fossil plants has been shown by Mr. Knowlton to confirm that of stratig- 

 raphy as to the distinctness of the Lax*amie and 1 >enver ton nations, and this 

 result leads to the hope thai when the Laramie fauna has been carefully 

 examined in respect to the distribution of its members all lines of evidence 

 may be found in accord. A few years ago the Laramie flora was deemed 

 as indivisible as is the fauna to-day. 



A study of the facts which have been presented brings out certain 

 features of the time-intervals between the close of the Laramie and the 

 close of the I Denver epochs which it may be well to recapitulate in this place. 



The pre-Arapahoe uplift.— The Arapahoe S('( 1 i 1 1 lei 1 1 S of tile Deliver liaMll 



testify beyond question to a preceding uplift which terminated Laramie 

 deposition in this vicinity. By this movement sonic adjacent area of 

 Mesozoic rocks was greatly elevated and was eroded to an unknown but 

 considerable extent before the beginning of deposition in the Arapahoe 

 lake or sea. This latter conclusion follows from the fact that the early 

 conglomerates of the Arapahoe contain pebbles from various horizons. 

 The Arapahoe sediments do not record a progressive erosion, cutting 

 deeper ami deeper into the uplifted Mesozoic area. SO much as they tell of 

 an already eroded surface. 



In Middle Park the entire Mesozoic section was upturned and eroded 

 prior to the Denver epoch. It is reasonable to suppose that this elevation 

 was contemporaneous with that of the Denver Basin. In the mountain 

 area tributary to the Canyon district the apparent evidence ot' the strata 

 described above indicates an uplift similar to that near Denver. In the 

 West Elk Mountains an uplift is les> certainly proven. From the general 

 character of the Laramie in Colorado, and its observed thickness, it may 

 even lie inferred that the pre-Arapahoe uplift terminated Laramie sedi- 

 mentation throughout the mountain district. No evidence is known to the 

 writer showing that Laramie deposition was ended by any other orographic 

 movement within the area of Colorado. 



