, PALEOZOIC TIME — CARBONIC. 635 



GENERAL DISTRIBUTION OF THE ROCKS OF THE ERA. 



The geological map on page 412, though small, is sufficiently detailed to 

 give a general idea of the distribution of the Carboniferous and Subcar- 

 boniferous areas of the eastern part of the continent. The former are 

 distinguished by doubly cross-barred marking ; the latter, which border 

 these, by singly cross-barred, with a cross in the small squares. The several 

 areas of the two combined formations are as follows : — 



I. The Acadian : covering part of western Newfoundland, of Nova Scotia, 

 and of New Brunswick. 



II. The Rhode Island : covering part of Rhode Island, and extending 

 northward and eastward into Massachusetts. 



III. The Worcester area : about Worcester, Massachusetts. 



lY. The Michigan area : occupying the larger part of Michigan between 

 the southern half of Lake Huron and Lake Michigan, having the coal- 

 measures over its central portion. 



Y. The Pennsylvania- Arkansas area : stretching in a zigzag way over 25 

 degrees of longitude and 12 of latitude ; first, from the southern border of 

 western New York, and a line just south of Lake Erie, to Alabama and 

 Mississippi; then, northward and westward to Illinois and Iowa; thence 

 southward and westward again to Arkansas and Texas. At the western 

 limit commences the '' Western Interior Sea," where the Carboniferous strata 

 pass out of sight beneath those of the Cretaceous. The coal-measures of this 

 area are mostly in three parts, underlaid and connected by the Subcar- 

 boniferous. These parts are thus separate, either because never united, or 

 more probably because of the removal of the coal-measures that once covered 

 the intermediate Subcarboniferous beds. 



YI. Over the Western Interior and along the summit region of the Rocky 

 Mountains, but without coal, and mostly as a limestone wherever there are 

 outcrops. 



YII. Along parts of the Great Basin, being a constituent of many of the 

 mountain ridges; also in the Sierra Nevada, and in other portions of the 

 Western border region. 



Ylll. In the Arctic regions, along a Avide belt between the parallels of 

 72° and 821°, northeast in course, from Banks Land on the west to Grinnell 

 Land on the east, and reaching beyond the latter to 83°, nearly the most north- 

 ern point of Arctic exploration. Also on Spitzbergen and Bear Island. 



The Coal-measures, or the areas of the Carboniferous period, have a smaller 

 range, and the productive Coal-measures, a still smaller. Of the above eight 

 regions, only numbers L, II., lY., and Y., to the east of the meridian 

 of 100° W., are coal-producing ; but the Arctic beds of Grinnell Land afford 

 coal, which may be available whenever the seas shall become navigable. 



The term Permo-Carboniferous is sometimes used for the beds of the Car- 

 boniferous and Permian periods of central and eastern North America, 

 because they make an essentially undivided series. 



