PALEOZOIC ROCKS AND ERA. 269 



feet. & In the former, sands, grits, and clays predomi- 

 nate ; in the latter, limestones. 3. In the former the 

 rocks are strongly folded ; these folds die out through 

 gentle undulations to level-lying strata in the latter. 4. 

 In the former the rocks are highly metamorphic ; in the 

 latter they are wholly unchanged. 



Area in the United States. 1. Eastern Paleozoic 

 Basin. The Paleozoic rocks cover a large continuous 

 area in the very best part of the United States. This area 

 is bounded on the north by the chain of the Great Lakes ; 

 on the east by the Blue Ridge of the Appalachian chain ; 

 on the south by a line running through mid- Alabama, 

 turning northward to the mouth of the Ohio Eiver ; then 

 south through mid-Arkansas and Indian Territory ; on 

 the west by the Western grassy plains. 2. Besides this 

 great area, there are several considerable areas scattered 

 about in the Plateau region and exposures along flanks 

 of mountains of the Plateau and Basin regions. 



Physical Geography. The physical geography of 

 the eastern portions of the North American Continent in 

 Paleozoic times can be made out with considerable cer- 

 tainty. In fact, we can in many places trace the Primor- 

 dial shore-line. Immediately in contact with the Canadian 

 Archaean on the north, and the Blue Eidge Archaean on 

 the east, are found patches, or continuous lines of a coarse 

 sandstone, which contain all the marks characteristic of 

 shore-lines, such as worm-tubes, worm-trails, crustacean 

 tracks, ripple-marks, rain-prints, etc. This is the old 

 Primordial beach. At the beginning of Paleozoic times, 

 therefore, the whole Paleozoic basin was covered by a sea 

 which beat against a land-mass to the north (Canadian 

 Archaean area), and a land-mass to the east (Blue Ridge 

 Archaean area). This is called the great interior Paleo- 

 zoic Sea. There was also a large land-mass in the Basin 

 region, and smaller masses, probably islands, in the Colo- 

 rado mountain-region, but the exact limits of these are 



