628 



GEOLOGY 



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The Mississippian and Pennsylvania!! sys- 

 tems of the west have commonly been grouped 

 together under the name Carboniferous. Most 

 of the formations of both systems are marine, 

 but sedimentation was somewhat generally 

 interrupted at the close of the Mississippian 

 period (p. 602). The formations of the Pennsyl- 

 vanian period are not in general so wide-spread 

 as those of the Mississippian, in this part of the 

 continent. 



In some parts of the west, the Carboniferous 

 system includes formations which resemble the 

 "Red Beds" of the next (Permian) system. 

 This is the case, for example, in the southern 

 part of the Rocky Mountain region, and in the 

 plains adjacent, and here the separation of the 

 Pennsylvanian system from the Permian is not 

 very distinct or has not been carefully worked out. 



The Carboniferous system of the west in- 

 cludes all sorts of sedimentary rocks, among 

 which are considerable thicknesses of limestone. 

 They are exposed at many points (Fig. 435) 

 and their existence over wide areas where 

 they are now covered by later deposits is 

 certain. The system is, however, not con- 

 tinous. Numerous islands of older rock prob- 

 ably maintained themselves throughout the 

 period, and a large area of land exi>tei! 

 throughout the Paleozoic era in western Nevada 

 (west of long. 117), and had an unknown 

 extension north and south. 



Many of the outcrops of the system in the 

 west are about areas of older rock, and it is 

 not always possible to tell whether these older 

 formations were islands in the Carboniferous 

 seas, or whether they once had overlying for- 

 mations, now removed. 



