J. H. Gilbert— Points in connection with Vegetation. 181 



The following conclusions are, I think, fairly deduced from 

 the facts as noted in Colorado: 



1st. In very early time in Colorado there was Archaean land 

 rising above the Paleozoic sea. As the Carboniferous age pro- 

 gressed this land diminished by encroachment of the sea, due to 

 subsidence of the land. This subsidence continued through 

 Triassic, Jurassic, and Cretaceous time into the early Tertiary. 



2d. At the close of the Lignitic there was a physical break 

 followed by a subsidence (at least locally) ami subsequently by 

 elevation, after the deposition of the Miocene strata. 



3d. The elevation of the Rocky Mountains as we now see 

 them in Colorado, is the result of an elevation commencing in 

 early Tertiary time, and continuing through the period, accel- 

 erated perhaps at the close of the Lignitic, and after the de- 

 position of at least Lower Miocene strata.* 



The elevation of the mountains was probably gradual as a 



It is an interesting fact that Colorado has a higher mean 

 elevation than any other State or Territory of the United 

 States,f and that we find there the highest mass of mountains, 

 and that the evidence points to the fact that in Paleozoic time 

 also we had here one of the highest areas, thus confirming 

 what Dr. Newberry has already intimated,:): that the outlines of 

 the western part !>f the Xorth American Continent were out- 

 lined from earliest Paleozoic time. 



Art. XXI- 



Dr. J. 



[Concluded from page 111.] 



Is the nitrogen combined under the influence of the soil with or 

 without the aid of manures, the son . ' ogen t— 



But if the plant itself cannot either assimilate free nitrogen, or 

 effect its combination so as to bring it into a state for its use. 

 may not such combination take place under the influence of 

 the soil ? 



More than thirty vears ago, Mulder argued that in the last 

 stages of decomposition of organic matter n 

 was evolved, and that this nascent hydrogen combined with 

 the free nitrogen of the air, and so formed ammonia. 



t See Ives Explor 



