STRUCTURE OF THE REGION. 233 



interior sea at the close of the Carboniferous age to the present date, the 

 A^ast region must have been nearly the same level lowland that it is toda}^ : 

 a plain whose surface has remained nearly at baselevel for ages, some- 

 times rising slightly, sometimes sinking a little, but never oscillating far 

 either one way or the other. 



Structure. — Great as is the difference between the broad central area 

 of low lying plains and its high serrated borders, there is a diversity of 

 structural features in the geologic details of each of the two districts as 

 distinct and as far removed from one another in character as are the two 

 widely separated types of surface sculpture. On the one hand, through- 

 out the marginal region of the interior basin the elevation of the land 

 has been accompanied by violent disturbances in the strata — folding, 

 crumpling, breaking, grinding the once horizontal beds until now they 

 lie at high angles, with upturUed edges everywhere exposed to the swift 

 ravages of nature's destructive agencies. The bold, rugged contours of 

 the mountain surface thus disclose the complicated structure of rocky 

 beds beneath. On the other hand, the lowland plain presents its strata 

 spread out in broad, nearly level sheets much in the same position as 

 when they were first laid down. Although made up almost entirely of 

 sediments dating back in their origin to old Paleozoic times, it is indeed 

 quite remarkable that, formed at a period remote even in geologic units 

 of time, the structural changes should be no greater than they are and 

 that the region should still retain over the greater part of its extent the 

 same simplicity of geologic structure that is found today among the 

 modern depositions of the coastal plains which fringe the great land 

 areas of the globe. The hypsometric changes over the whole region 

 have been, therefore, of the character of continental elevation and de- 

 pression. 



Geologic Provinces. — The present boundaries of the Mississippi valley 

 form approximately the limits of an area which in Carboniferous times 

 had a development peculiarly its own and in a great measure unaffected 

 by events transpiring in neighboring districts. The origin and deposi- 

 tion of its strata, the lithologic characters of its beds, and the succession 

 and evolution of its faunas were wholly independent of the surrounding 

 areas. In short, the Carboniferous basin of the Mississippi valley repre- 

 sents, in every sense of the word, what in geology is called a " geologic 

 province." 



Character of Deformations. — The great economic value of the coal-bear- 

 ing formation of the Mississippi province has directed particular atten- 

 tion to its geology. Within the limits of the region it may now be said 

 that the Coal Measures have received more careful attention than any 

 other of the geologic formations represented ; but at the same time, for 



