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1906] SHANTZ—VEGETATION OF THE MESA 19 
istic stamp to the vegetation; socéeties, or minor divisions, char- 
acterized by principal species and dominant usually over smaller 
areas and only during the aspect in which they occur; communi- 
ties, or smaller associations, usually of secondary species. 
GEOLOGY. 
The eastern base of the Rocky Mountains shows a great many 
rock systems which are upturned and all come to the surface in or 
near this region. Lying on the Archean granite, which forms the 
mountains at the west, is found a Cambrian red sandstone, gravel 
or lime. The Silurian or Manitou limestone lies next, followed by 
the reddish gray quartzite sandstone of the Carboniferpus. East of 
this the Garden of the Gods is formed by the great red sandstone out- 
crop which is placed in the Permian or in the Triassic—authorities 
differ. The Jurassic, which lies next, is followed by the Cretaceous 
rock system, represented by the following epochs. The lowest is of 
the Dakota—the white sandstone ridge or hogback, and the great. 
sandstone ledges. Lying next and buried in most places under. 
the talus of the latter is the Benton shale. The most eastern of 
the series of hogbacks marks the outcrop of the Niobrara limestone. 
The Fort Pierre shale is found in many places east of the lime ridge 
and underlies the whole Mesa region. Lying above this is found 
the recent Quaternary deposit of gravel of granitic origin. 
The sedimentary deposits underlie the entire Great Plains region, 
but in most places are covered by the more recent wash from the 
mountains. For the geological development and structure of the 
Great Plains, as well as for a description of the topography and 
climate, the reader is referred to the exceedingly interesting publi- 
cation by JoHNson.? The following quotation from this source 
(p. 612) gives a very clear idea of the origin of the plains. 
The Great Plains are of such vast dimensions it is only in imagination that 
they can be regarded as a foot slope to the Rocky Mountains. However, in 
the sense that, superficially, ranging down to several hundred feet in depth, they 
have been built to a smooth surface by mountain waste, stream-spread to great 
distances, they have this character. At the base of the mountains the Plains 
mass has a thickness, to sea level, of several thousand feet. It is made up in 
2 Jounson, W. D., The high plains and their utilization. Ann. Rept. U. Ss. 
- Survey 214: 601-741. 1899-1900. 
