284 -. P.. EMMONS — OROGRAPHIC MOVEMENTS. 



often obscure in a given section, and can only be accurately determined by 

 a careful stratigraphical study of a considerable area. This is well illus- 

 trated in the case of the Denver region, of which a tnosl exact and detailed 

 survey has been made recently under my supervision by Messrs. Cross and 



Eldridge. They have shown that, since the movement at the close of the 

 Laramie proper, there have been deposited upon its eroded surface two 

 succeeding series of beds, of a thickness of 800 ami 1,400 feet respectively, 

 called the Arapahoe and Denver formations, the former of which was up- 

 lifted and eroded before the deposition of the latter. The greal Lengtb of 

 time that must have elapsed subsequent to the post-Cretaceous movement is 

 proved by the fact that the Arapahoe formation is made up of material 

 recognizable as derived from different horizons of the 14, oho odd feet of 

 Mesozoic beds upturned by it. including the Laramie. It is further empha- 

 sized by the composition of the beds of the Denver formation, which are 

 largely, and in their lower portion almost exclusively, made up of debris 

 of a very great variety of andesitic rocks, none of which could be found in 

 the lower beds and the source of which has not yet been discovered in the 

 adjoining regions, showing that the interval must have been of sufficient 

 length to admit of the outpouring of a great variety of andesitic rocks and of 

 their almost complete denudation before the close of the Denver period. 



in earlier examinations of the region, on account of the peculiarly com- 

 plicated structural conditions, all these beds had been assumed to belong to 

 one conformable series, and the plants collected from the Laramie beds and 

 from the Denver beds above are indiscriminately designated by Pro- 



- ir Ward, in his Synopsis, as '-from the Laramie at Golden," although 

 I had previously called his attention to our discovery of the unconformity 

 and pointed out the differences in the matrices of the respective specimens in 

 his collections. 



With regard to the age which would properly be assigned to these 

 I ater beds from a palseontological point of view — thai is, as determined by the 



general laws of BUCCession of animal and plant life, which the pit-sent 

 knowledge of the development of life in Mesozoic ami Tertiary times in 

 other part- of the world have led biologists to make, —there exists considera- 

 ble uncertainty. Of the organic remains thus far discovered neither plants 



nor invertebrates can be considered of sufficient tax >mic value to afford 



decisive evidence as to their Cretaceous or Tertiary age. The vertebrate 

 remain-. on the other hand, present the m aresl analog; to a recently described 

 vertebrate fauna, assigned by its discoverer to the Laramie Cretaceous. No 

 published evidence exists of the stratigraphical or structural relation- of the 

 bed- in which these occur; only the hare statement of the author that they 

 belong to the Laramie. Furthermore, il is known that some of the beds, 

 who-e fauna i- -aid by palaeontologists to have a Laramie facies, are dis- 



