406 



thoroughly impressed with the Cretaceous age of these beds, while those 

 who have studied them from the north and northwest toward the interior 

 basin, received their first impressions that they were of Tertiary age. We 

 now see that, taking a view of the entire West, the coal does not seem to 

 be confined to any period, but even extends down into the Jurassic. Dr. 

 Ifewberry, as far back as 1859, discovered coal in beds of undoubtedly 

 Jurassic age in Arizona, as his section of the strata near the old Moquis 

 villages would indicate. 



If it is true that, taking into view the entire Lignitic area of our West- 

 ern Territories, the coal-beds are continuous in every division, from the 

 Jurassic to the summit of the Upper Lignitic, we might make this gen- 

 eral division : 1st. Lower Lignitic group, including all the Lignitic de- 

 posits of marine origin ; 2d. Middle Lignitic, embracing all deposits of 

 brackish- water origin; 3d. Upper Lignitic, including all beds of purely 

 fresh-water origin. In my opinion, the first division would include all 

 beds to the summit of the true Cretaceous ; the Middle Lignitic embraces 

 my Transition series, or, if they are not admitted by geologists, I would 

 insist upon their Lower Tertiary age. The Upper Lignitic, or fresh-water 

 deposits, are of unquestioned Tertiary age. The details upon which these 

 opinions are founded have been printed in my former reports, and cannot 

 be repeated here without rendering this article too long. 



On the west side of the Laramie Mountains, in the Laramie Plains, 

 the Lignitic group re-appears to the westward. It is well shown at Eock 

 Creek, on the east side of the Medicine Bow range, and is again well 

 developed at Carbon, on the Union Pacific Eailroad, where the coal has 

 been very extensively wrought. Tracing the underlying formations from 

 the west base of the Laramie range westward, across the plains, we find 

 the full series of the Cretaceous groups gradually overlapping each other^ 

 and, before reaching Eock Creek, the lower beds of the Lignitic group 

 extending over the Cretaceous beds, and corresponding with the same 

 group on the east side of the Laramie Mountains. 



The lithological characters, the fossils, and the relations of the group 

 to the main beds below are so similar that I have inferred that all 

 these beds once extended without interruption across the area now occu- 

 pied by the mountain-range, and I still believe this generalization to 

 be true. It is quite possible that some of the loftier ranges, as those of 

 which Long's Peak and Pike's Peak form a part, were outlined far back 

 in the past; but many of the minor ranges were undoubtedly elevated 

 at a comparatively modern period. 



As we proceed westward, there seems to have been an enormous 

 thickening of the sediments, particularly during the Cretaceous and 

 Tertiary periods. Commencing near the point where the Union Pacific 

 Eailroad crosses the !N^orth Platte, or about meridian of 107°, far west 

 and south into the great interior basin, the aggregate thickness of the 

 Cretaceous and Upper Lignitic (not including the fresh-water basins) 

 must be at least 12,000 feet. As we reach the interior basin, all the 

 formations known in that region have attained an enormous thick- 

 ness, so that Mr. Clarence King regards the aggregate thickness of the 

 sedimentary rocks in Utah at 56,000 feet ; while in the immediate vicin- 

 ity of the eastern division of the Eocky Mountains, the entire thickness 

 of the sedimentary deposits cannot be one-third that amount. It is in 

 the immediate valley of the Colorado, in Western New Mexico and Ari- 

 zona, in the great basin between the Sierra and the Wasatch Mountains, 

 where the principal thickening of these formations occurs. In such cases, 

 their external characters change much, and the variety and quantity of 

 the organic remains diminish to a great extent. Coarse conglomerates 



