THE PENNSYLVANIAN PERIOD 163 



system which is usually thinner and less arenaceous than in the 

 Appalachian district. 



Small areas of Pennsylvanian igneous and metamorphosed 

 sedimentary rocks, together with some graphitic coal, occur in 

 Rhode Island and Massachusetts. 



Coal-bearing strata of this age attain a thickness of thousands 

 of feet in New Brunswick and Nova Scotia. 



Pennsylvanian Rocks in the West. — From the Rocky Moun- 

 tains westward in the United States, the Pennsylvanian rocks are 

 practically all of true marine character and consist largely of 

 limestone and sandstone but entirely without coal, thus being in 

 marked contrast with the rocks of the system in eastern North 

 America. 



Thickness of the Pennsylvanian. — In the Appalachian dis- 

 trict, the system ranges in thickness from about 1000 feet to 

 fully 7000 or 8000 feet. A maximum thickness of 13,000 feet is 

 known in Nova Scotia, and 12,000 feet in Rhode Island. Through 

 the Mississippi Basin the thickness is usually not more than 1000 

 to 2000 feet, though in Arkansas a thickness of over 18,000 feet 

 has been claimed. In the western United States the thickness 

 varies much, though it is usually at least several thousand feet. 



Igneous Rocks. — Considerable amounts of granite are in- 

 truded into the Pennsylvanian and other rocks of Massachusetts, 

 but these may really be of Post-Pennsylvanian age. Also in the 

 Cordilleran region from northern California to Alaska we learned 

 that vulcanism occurred on a large scale in Mississippian time, and 

 it is probable that it continued into Pennsylvanian time. 



Physical History 



Early Pennsylvanian. — As we learned in the preceding chap- 

 ter, the Mississippian period closed with a widespread emergence 

 of all (or nearly all) of the submerged areas in eastern North 

 America. Very early in the Pennsylvanian the sea began to trans- 

 gress over the land by extending a long, narrow estuary north- 

 ward through the Appalachian district as far as Pennsylvania. 

 The Pottsville sandstones and conglomerates, derived by erosion 

 from Appalachia immediately to the east, were accumulated to 

 great thickness in this estuary, and it is thus readily seen why the 

 Pottsville should be thickest on the east side. Gradually the early 



