INTRODUCTORY REMARKS. LVII 



regarding the plants as Miocene types, which opinion was at one time adopted 

 with regard to the age of this group by Dr. Hay den and the writer in the 

 following words : 



'So far as we have been able to compare them [the invertebrate fossils 

 from these rocks] with figures and descriptions of foreign species, the 

 evidence appeared contradictory, some of them being like Miocene and 

 others like Eocene types. As we now have, however, the additional weight 

 of evidence derived from Dr. Newberry's Investigations of the fossil flora of 

 these formations in favor of the conclusion that they are of Miocene age, we 

 can no longer hesitate in referring them to that epoch."* 



On Dr. Havden's subsequent discovery, however, that these beds pass 

 at some place beneath the White River group, generally regarded as' 

 Miocene, we adopted the conclusion that the Fort Union group is proba- 

 bly Lower Eocene; and the writer cannot remember that either of us 

 has departed from this opinion, though lie has never hesitated to mention any 

 facts, when met with, that might seem to cast even slight doubts upon such 

 a conclusion. 



Professor Lesquereux, who has devoted much time and study to the 

 investigation of the fossils from this group of the Upper Missouri rocks, from 

 much more extensive collections than those examined by Dr. Newberry, pro- 

 nounces them decidedly Lower Eocene. 



Undoubtedly, confusion has been created by some who have discussed 

 the question of the age of the Lignite formations of the far-West by referring 

 to such deposits in the Upper Missouri, and all over the Rocky Mountain 

 region, as well as away on Vancouver's Island, as if they must of necessity 

 all be of one age. They seem to think that to show that the lignites at 

 some places are Cretaceous or Tertiary, as the case may be, is a strong 

 argument that all are the one or the other, according to their own view of the 

 subject. The fact is, however, the presence or absence of lignite proves 

 nothing of itself, as lignite undoubtedly occurs in both Cretaceous and Ter- 

 tiary rocks in the far-West. In Western Colorado, Mr. Holmes and Dr. 

 Peale found lignite beds all through the well-marked Cretaceous rocks. 



Some, who advocate the conclusion that all of these lignites are of Cre- 

 taceous age, refer to the well-known fact that the Coalville mines in Utah 

 and the coal-beds at Bear River, Wyoming, as well as others in Colorado 



• Proceed. Acad., Nov., 1856, 268. 

 H — VIII 



