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J. D, Dana on Time-boundaries in Geological History. 227 
Art. XXI.—On the Appalachians and Rocky Mountains as Time- 
boundaries in Geologiwal History; by JAMES D, Dana. 
limits of the vast interior continental basin. All other lines of 
heights are small in comparison. 
the oceans and the continental interior. The three eras, after 
the Azoic, recognized by geologists, are the Paleozoic, or ancient 
time, the Mesozoic, or medieval time, and the Cenozove, or recent 
time; the first and second having their intervening limit be- 
tween the Carboniferous and Reptilian ages, and the second and 
third, between the Cretaceous period closing the Reptilian age 
and the Tertiary commencing the age of Mammals.’ w, the 
elevations of the two mountain chains, referred to, date from the 
limits of these eras. At the first of these limits, or as the clos- 
ing act in Paleozoic history, the rocks of the Appalachian re- 
T " . 
Cenozoic time, the mass of the Rocky Mountains began to rise 
above the ocean. 
Prof. Agassiz, in a recent rin the Atlantic Monthly, places the close of the 
Paleozoic ny Sea Downie sol the writer's view, the whole bearing of the 
science is against any such new arrangement of the Geological ages. 
