FORMATIONS AND PHYSICAL HISTORY 



447 



'nlumbiu. West of this range, the system includes much vol- 

 canic rock, the greater part of which was extruded before the 

 close of the period. The system is continued northward into 

 Alaska, 1 where it is less widespread than the Mississippian, so far 



J 

 Mis 



Fig. 39 1. Section showing the position and relations of the Carboniferous sys- 

 at a point in Colorado. J #? = Archean; - = Cambrian; = Ordovician; M = 

 ississippian; Cw and Cm = Carboniferous; / = Jurassic; Kd, Kb, Kn, and Km = 

 Crciai (.-tins. Length of section about 6 miles. (Eldridge, U. S. Geol. Surv.) 





mdl 



ig. 392. Section showing the Carboniferous in the Sierras of central Cali- 

 >rnia. C' = Carboniferous; / (Mariposa slates) = Jurassic; wwfi = metadiorite; 

 ann = Ami>hibolite schist; 7V r = igneous rock of various sorts, of Neocene age. Length 

 of sci tion about 6^3 miles. (Ransome, U. S. Geol. Surv.) 



as present knowledge goes. In the Arctic lands of America, the 

 Mississippian and Pennsylvanian are not differentiated. One or 

 both are widespread. 



Thickness. The thickness of the system has a wide range, but 



like all preceding systems of the Paleozoic, it is thick (4,000 to 5,000 



in the Appalachian Mountains. In the interior, it exceeds 



1,000 feet in but few places; but in Arkansas, the Coal Measures 



have been assigned the remarkable thickness of more than 18,000 



feet, from which it is inferred that there must have been land close 



at hand capable of supplying sediments in great quantity. This was 



>robably the axis of the Ouachita uplift. In Texas, the thickness of 



le system ranges up to 5,000 feet, and in the west it is even thicker. 



Coal 



The general conditions under which sandstone, shale, and lime- 

 >ne originate have been outlined, but there has been no occasion 

 ;retofore to consider the formation of coal. From its economic 

 iportance, coal has been studied with more care than most sorts 



1 Brooks, Professional Paper 45. 



