870 THE JOURNAL OF GEOLOGY. 
some greatest revolutions which have in some way directly or indi- 
rectly affected the whole earth, and which may therefore be used to 
form the basis of the primary divisions of time.’ These he calls 
“‘Critical Periods”’ in the earth’s history. The marks of such periods 
are readjustments of the crust of the earth, causing widespread changes 
in physical geography, affecting profoundly the climate of the whole 
earth, and causing marked changes in organic forms. These are indi- 
cated in the rocks by (1) widespread unconformities; (2) great, gen- 
eral, and rather sudden changes in organic forms; (3) the introduction 
of new and higher dominant classes; (4) the birth of great mountain 
ranges. Of these periods, beginning with the last, he enumerates the 
following—the Glacial revolution, the post-Cretaceous or Rocky 
Mountain revolution, the post-Palzozoic or Appalachian revolution 
and the pre-Cambrian revolution. 
Of these it may be said—-(1) they become shorter and shorter as 
we progress in the earth’s history ; (2) the effect of the introduction of 
new dominant types in producing changes in the whole organic king- 
dom steadily increases; (3) the oscillations of temperature have gradu- 
ally increased. These periods hasten the steps of evolution and 
increase organic diversity, but diminish geographical diversity. These 
revolutions seem to be contrary to uniformity in the forces and laws of 
nature, but they are not so in reality. We may conceive of phenomena 
as being under the influence of two opposite forces, one progressive, 
the other conservative. Such will be more or less paroxysmal. Resist- 
ance at first prevails, and there is little change, but forces of change 
are accumulating, and finally resistance gives way and conspicuous 
changes take place rapidly. 
Leepee Buttes. By G, K. GitBert and F. P. GuLLiver. 
In the Bulletin of the Geological Society of America, Gilbert and 
Gulliver have some interesting things to say of the Teepee Buttes of 
Colorado. They are found ina belt about ten miles wide and fifty to 
sixty miles long, at least, beginning at Little Butte Station on the 
Denver & Rio Grande Railroad, and extending to the south and east 
along that railroad. They are not continuous but gravel areas separate 
areas dotted with them. They are twenty to thirty-five feet in height, 
rudely circular, and diameter is usually less than height. The struc- 
ture is peculiar, viz.,a core of coarse, light-gray limestone, bedded 
and full of marine fossils, surrounded by a shale which has few fossils 
