LETTER TO THE SECRETARY. V 



firmed, not by the discovery merely of one or two doubtful species common to 

 the strata of each of these regions, but by an identical Molluscan fauna ranging 

 through the whole series in each of the regions named. This shows that the 

 strata referred to, all belong to one well-marked period of geological time. 

 Dr. White arrives at these conclusions, not merely because there is a similarity 

 of type in the fossils obtained from the various strata of the Laramie Group 

 with those that were before in question, but by reason of the specific identity 

 of many fossils that range from the base of the Laramie Group up into and 

 through the strata that were formerly referred to the base of the Wahsatch. 

 Some of these species were found by Dr. White in the Laramie strata on 

 both sides of the Rocky Mountains, with a vertical range of not less than 

 three thousand feet and a geographical range of more than a thousand miles. 

 The conclusion, therefore, becomes more and more apparent that while 

 the principal groups of the Mesozoic and Cenozoic formations in the West 

 have each peculiar characteristics, and are readily recognized by the geologist, 

 they really form an unbroken series of strata, not separated by sharply 

 defined planes of demarcation, either stratigraphical or palseontological. The 

 facts as we understand them at the present time would seem to warrant this 

 general division, viz.: a marine series, Cretaceous; gradually passing up into 

 a brackish-water series, Laramie; gradually passing up into a purely fresh- 

 water series, Wahsatch. It is also probable that the brackish-water beds 

 on the Upper Missouri must be correlated with the Laramie, and that the 

 Wahsatch Group as now defined and the Fort Union Group are identical 

 as a whole, or in part at least. The plants which are recorded in this volume 

 began their existence at the base of the Laramie Group, and continued 

 through the entire series, brackish and fresh-water. The reason will now 

 become apparent why I have, in my former reports, called the Laramie 

 Group a transition series, or beds of passage, not as a distinctive name, 

 but only as indicating the fact that they seemed to bridge over the chasm 

 between the purely marine Cretaceous and the purely fresh-water Tertiary. 



The lack of animal remains in the Lower Lignitic Measures, especially 

 those of Colorado, is remarkable. On the other hand, all the coal-bearing 

 strata above the Cretaceous Fox Hills Grou|i abound in well-preserved vege- 

 table remains. The comparatively few specimens of fossil plants obtained 

 by the Survey in Colorado up to the year 1870 pointed to the conclusions 



