June 18, 1875.] ^41:/ [Stevenson. 



THE GEOLOGICAL RELATIONS OF THE LIGNITIC GROUPS. 

 By John J. Stevenson, 



FROFESSOB OF GEOLOGY IN THE UNIVERSITY OF THE CITY OF NEW YORK. 



{Read before the American Philosophical Society, Jihne 18, 1875.) 



The principal lignitic areas of our country are two ; one on the Pacific 

 Coast, extending in all from Alaska to Lower California ; the other in the 

 Rocky Mountain Region, stretching from the Arctic Ocean to New ]\Iex- 

 ico. Between the Sierra Nevada and the Rocky Mountains no lignites 

 have been discovered. 



Within a few years the controversy respecting the geological relations 

 of these lignites has become very keen, some regarding them as Creta- 

 ceous, others as Tertiary. In many instances, the conclusion reached by 

 investigation of the flora is directly contradictory of that reached by 

 study of the fauna. Over a considerable portion of the Rocky Mountain 

 Region the rocks of the Great Lignitic Group are barren of animal re- 

 mains and only plants are found. Where the fauna is seen the genera 

 and species are usually Cretaceous, and where they are not clearly so 

 they are fresh-water, and therefore of little value either way. The flora 

 is very closely allied in general character to the Tertiary flora of Europe, 

 many species in each being apparently identical. 



During my connection with Lieut. Wheeler's Expedition, I passed over 

 a portion of the disputed ground, and so became involved in this contro- 

 versy. I have thought it necessary to study with care all the material 

 within my reach which seems to bear upon the subject. While this study 

 has shown me that the question at issue is by no means so simple as I 

 supposed it to be, when I rendered my report to Lieut. Wheeler,* yet it 

 has confirmed me in my conclusion there given, that the Great Lignitic 

 Group, or the Fort Union Group of Hayden, is Cretaceous and not Eo- 

 cene. 



It is essential here to determine the value respectively of the various 

 forms of geological evidence, for all have been cited in this discussion, 

 and in some respects they seem to be contradictory. 



In every case where applicable, stratigraphy is final. So long as we 

 can trace a rock continuously we have no doubt of its identity. But 

 stratigraphy in this simple form is not often available to any great extent. 

 So variable are the rocks in large areas, owing to the different conditions 

 under which matter may be deposited synchronously at distant localities, 

 that direct comparisons of sections by lithological characters, or even by 

 tracing, becomes impossible. We are compelled, therefore, to resort to 

 palasontology in addition. Our geological column is based upon the suc- 

 cession of the marine invertebrata. 



The stratified rocks, with the exception of comparatively insignificant 

 portions, were deposited under the ocean, and of those which contain the 

 remains of terrestrial organisms, by far the greater proportion was formed 



* June, 1874. 

 A. P. S. — VOL. XIV. 3f 



