THE MISSISSIPPIAN PERIOD. 507 



Igneous Activity. 



During this period, according to present determinations, there 

 was great igneous activity in the west. West of the Gold ranges 

 in British Columbia, the early Carboniferous is made up largely of 

 igneous rock, with intercalated beds of clastic sediments. The area 

 affected by vulcanism at this time, or soon after, extended from Alaska 

 on the north to California on the south. 1 For this region the vol- 

 canic activity subsided before the close of the Pennsylvanian period. 



Close of the Period. 



Geographic changes which contracted the eastern interior sea 

 to very narrow limits (Fig. 240), if they did not completely obliterate 

 it, brought the Mississippian period to a close. Similar changes can- 

 not be affirmed to have affected the western half of the continent to 

 the same extent, but the tendency of recent studies is to show that 

 geographic changes of consequence took place even in this part of 

 the continent at this time. Evidence of this import is found 4n Mon- 

 tana 2 and farther north, 3 where the Lower Carboniferous is more 

 wide-spread than the Upper, in British Columbia, 4 in Colorado, 5 in 

 Indian Territory, 6 where there was perhaps notable warping, and 

 perhaps in Utah. 7 



In some parts of the west, however, so far as now known, marine 

 conditions prevailed uninterruptedly from the early Mississippian 

 period to the later part of the Pennsylvanian period. 



Sections of the Mississippian system, 8 at various points in the United 



1 Dawson, op. cit., p 85. 



2 Montana folios, U. S. Geol. Surv. 



3 Dawson, Bull. Geol. Soc. Am., Vol. XII, p. 85. 



4 Dawson, Science, March 15, 1901. 



5 Crested Butte quadrangle of the Anthracite-Crested Butte folio, U. S. Geol. Surv. 

 See also 5 below. 



8 Colgate, I. T. folio, U. S. Geol. Surv., p. 5. 



7 Emmons, Oogenic Movements in the Rocky mountains, Bull. Geol. Soc. Am., 

 Vol. I. Also Mono. XXVII, U. S. Geol. Surv., and Anthracite-Crested Butte folio, 

 U. S. Geol. Surv. 



8 The Mississippian system is not represented as a separate system on the folios 

 of the U. S. Geological Survey; but in the later folios, such as the Indiana, Latrobe, 

 Elkland-Tioga, Masontown-Uniontown, and Brownsville-Connellsville folios of Pennsyl- 

 vania, the Gaines folio of Pennsylvania-New York, the New Castle folio of Wyoming- 

 South Dakota, and the Edgemont folio of South Dakota-Nebraska, it is represented 

 as a major division of the Carboniferous. In the text of older folios the Mississip- 



