PALEOZOIC TIME — CARBONIC. 



633 



NORTH AMERICA. 



Topography. 



The topography of the continent at the commencement of this era is 

 approximately represented on the accompanying map, Fig. 990, on which the 

 dotted lines over the surface, marking river courses, outlines of lakes, etc., 

 are to be taken only as indicating positions. The chief change since the 

 commencement of the Upper Silurian (page 536) is in the eastern portion — 

 or that of the Eastern Interior or Great Northeast Bay, which, at the opening 

 of the coal era, was a complete bay in outline, reaching northeastward to the 



990. 



Map of part of North America at the commencement of the Carbonic era. 



boundary of northeast Pennsylvania. It was in fact a double-headed bay, a 

 branch passing northwestAvard from the Pennsylvania portion or bay (P), over 

 Michigan, and making thereby a Michigan Bay (3/). The Cincinnati Island 

 (C) became part of the mainland, while the Tennessee was submerged. In 

 addition, the Connecticut valley trough and the St. Lawrence valley trough 

 were probably above the reach of salt water, or, at least, were not subsiding 

 troughs, for no Carboniferous rocks occur within them; they were, probably 

 the courses of fresh-water streams. But the Gaspe- Worcester trough must 

 have been an open channel, southward to Worcester at least, and the Acadian 

 trough, from western Newfoundland to Narragansett Bay, was a still larger 



