SUEVEY OF COLORADO AND NEW MEXICO. 89 



more and more important. The amount of labor that is continually 

 expended in opening mines and driving tunnels is immense, and their 

 future importance as a source of wealth to the country greatly in- 

 creased. The same remarks will apply to the gold mines of Gilpin 

 Count^^ There are some remarkably rich lodes which have yielded the 

 enterprising miners untold wealth, and some that will continue to do so. 

 In the majority of cases, where proper management and economy have 

 been employed, the mines have been a great source of profit to the 

 miner. It is not necessary to enter into the causes of the wonderful 

 failures and swindling operations which have brought Colorado into 

 such disrepute in the past. It is sufficient for me to state my belief that 

 the mining districts of Colorado will yet be regarded as among the richest 

 the world has ever known. 



CHAPTEE XII. 



REVIEW OF LEADING GROUPS, ETC. 



This final chapter to my report, which I have added here, will contain 

 a brief review of the leading groups of strata noticed in this and my 

 previous reports, as well as a few additional observations and chemical 

 analyses. The details of my labors will be presented in my final reports 

 at some future period. 



I have already alluded to my belief that this western country during 

 the tertiary period was covered to a greater or less extent with a chain 

 of brackish or fresh-water lakes; that the tertiary period began its 

 existence with brackish water deposits, which gradually became fresh 

 water, and thus continued up to the present time. It is hardly possible 

 to synchronize all these groups of strata with our present knowlege ; 

 but in order that our efforts in that direction may be facilitated, I have 

 thought it best to give them specific names, which may be regarded 

 provisional for the present. Each one of these groups will doubtless 

 afibrd a flora and fauna to a certain extent j)eculiar to itself, and a 

 greater importance will be attached to it when grouped around some 

 specific names. 



Proceeding southward from Cheyenne we pass over the coal forma- 

 tions of the tertiary x)eriod, which have already been called, on the 

 Upper Missouri, the Fort Union group. This group I regard as marking 

 the dawn of the tertiary age in the West, and as covering a far more 

 extended area than any other group of this epoch. It is continuous 

 southward from the Missouri Yalley to Colorado, interrupted only by a 

 belt of White Eiver beds about two hundred miles wide. I think these 

 beds also extend far northward into the British possessions, probably 

 nearly or quite to the Arctic Sea. 



About forty miles south of Denver we have a high divide, or ridge, 

 which forms a sort of water-shed between the Platte and Arkansas 

 Kivers. This is comi^osed of a group of strata, mostly sandstones and 

 sands jutting up against the mountains in a slightly disturbed position 

 and not conforming to the older rocks. These beds are undoubtedly 

 middle tertiary, and I have called them the Monument Creek group. 



I do not think that such terms as eocene, miocene, pliocene, &c., are 

 at all applicable to the tertiary deposits of the West, and I therefore 

 designate them as lower, middle, and ui)per tertiary. I regard all the 

 coal beds of the West as lower tertiary. It is true that some of these 



