10 UNITED STATES GEOLOGICAL SURVEY— TERTIARY FLORA. 



shafts, near Plattevillc, north of Denver, have also reached coal quite near the 

 surface, showing that the belt of the Lignitic extends, locally at least, to a 

 great distance eastward from the base of the mountains. 



§ 2. — Slratigraphi/ of the Lignitic and i/s capacihj for combustible mineral. 



On this subject we have documents more precise than for the former, 

 though they are not complete as yet ; for the amount, thickness, and chemi- 

 cal value of the coal or Lignitic beds is tar from being exactly known, or 

 even far from being possibly estimated. Where the Lignitic has been recog- 

 nized from its base, it has been seen overlying the Upper Cretaceous strata, 

 whose section is exposed in the Annual Report of Dr. Hayden for 1870, p. 

 87. The two upper groups, the Fort Pierre group. No. 4, and the Fox Hill 

 bed. No. 5, have generally an abundance of invertebrate fossil remains, and a 

 peculiar lithological composition, which makes them easily recognizable. In 

 the North Basin, or the Fort Union group, the superposition of the Lignitic 

 to the Cretaceous is not marked by any definite line of demarkation. Indeed, 

 this line is seen nowhere, neither in an abrupt change of the compounds, nor 

 in an unconformable stratification, nor in the character of the faunas. On this 

 subject, Dr. Hayden remarks :* — "When we bear in mind the fiict that where 

 the Lignitic has been seen in contact with the last Cretaceous beds, the two 

 have been found to be conformable, however great the upheavals and the dis- 

 tortions may be, while at the junction there seems to be a complete mingling of 

 sediments, one is strongly impressed with the probability that no important 

 member of either system is wanting between them." And at another pagetf — 

 "That the passage from the brackish- to the fresh- water beds of the Tertiary 

 is not marked by any material alterations in the nature of the sediments, nor 

 have we, as far as it is known, any reason for believing that any climatic or 

 other important physical changes, beyond the slow rising of the land and the 

 consequent recession of the salt and brackish water, took place during the 

 deposition of the whole of th(! oldest members of the Tertiary." 



In his Geological Report on the P^xjdoration of the Yellowstone and 

 Missouri Rivers, IS.oO-fiO, Dr. Hayden remarks upon the Lignitic of the 

 Yellowstone River (p. 58) : — " Passing up the valley of the Yellowstone, we see 

 the gray sandstone Tertiary, which we have o])served to cover the Cretaceous 

 nearly to the foot of the blulis. The junction of these formations is quite 



"Annual Kcpoit, 1874, p. '2-1. t Same Rcpml, p. a'>. 



