ENDLicH] CONCLUSION — TERTIARY — ^VOLCANIC FORMATIONS. Ill 



group. Nowhere have I seen it, however, giving promise of remunera- 

 tion in case it were worked. No doubt beds may be found within the 

 districts explored that would eventually furnish an acceptable yield, in 

 case the demand for coal should justify their development. So long as 

 not even the coal of the Laramie age is utilized to its full capacity, there 

 will probably be no occasion to seek in the Wahsatch Group for fuel. 



Green River Group (Miocene). — This group of the Tertiary I have met 

 with in Colorado only along the White Eiver. There it is very typically 

 developed. Great thicknesses of hard shales, of white, yellowish, and 

 greyish color, denoting fresh-water deposits, characterize the group. 

 It has been fully discussed in the preceding pages of this report, and 

 but little remains to be said. During my examinations! have not found 

 the Green River beds so interesting palseontologically as it is in many 

 other regions. There the large number of fossil species found invest 

 the formation with great interest. As is the case with nearly all groups 

 of similar or analogous genesis, certain regions were by far better sup- 

 plied than others with vegetable and animal life during the period of 

 deposition. Thus we may find that the flora or fauna, as preserved 

 in the shales, may be one of exceedingly great interest at one locality, 

 while but a short distance off, comparatively speaking, there is scarcely 

 a plant or an animal remaining whereby to identify the strata. 



With the Green River Group the list of sedimentary formations that 

 I found in Colorado is exhausted. The local deposits termed " lake- 

 beds" more properly belong to the category of drift than of sediment- 

 ary formations. 



VOLCANIC FORMATIONS.* 



In Colorado we find several distinct types of volcanic formations. 

 Through advantageous grouping, their correlation and relative ages 

 can readily be determined. During the first three years of my work 

 I encountered large masses of volcanic rocks of various types. Their 

 recognition was a comparatively easy matter, after a classification had 

 been decided upon. Four main divisions may be distinguished: 



Trachorheites, 

 Porphyritic Trachytes, 

 Basaltoids, and 

 Dikes. 



Of these the last named is but a varying form of occurrence of any of 

 the preceding divisions. It has, for that reason, been placed at the end 

 of the enumeration, which is in chronological order, with this exception. 

 Each of the divisions will be discussed briefly, and any one wishing 

 additional information upon the subject may be referred to the annual 

 reports and to the shortly forthcoming paper. 



Trachorheites. — This division comprises the four groups of Richthofen : 

 Propylite, andesite, trachyte, and rhyolite. Though each one of these 

 i« well defined and characteristic in itself, it is impossible to distinguish 

 them at all times during the work in the field. Therefore they have been 

 designated collectively by the above-given name. 



Propylite, which is the oldest of the series, I have not recognized in 

 Colorado. It may be that it blends too closely into the andesite, or 

 even trachyte, or it may be wanting altogether. It must be stated 

 that all the outpourings of lava in Colorado belong to the type that has 

 been designated as " massive" in contradistinction to *' volcanic" erup- 



* I am at present engaged upon a paper on the " Volcanics of Colorado.'' Therein 

 all the correlations and special features shall be set forth at greater length than 

 here. — E. 



