POTSDAM PERIOD. 197 



materials of these layers were the moving sand and pebbles of the 

 shore. 



(2.) Interior basin. — South of this northern line, over the interior 

 continental basin, whatever Potsdam rocks exist are mostly con- 

 cealed from view ; so that we fail of direct evidence as to the depth 

 in that region. The rocks in Texas, however, bear the same testi- 

 mony as those of the North. But we have indirect evidence with 

 regard to the depth of the interior basin, in the facts observed 

 along the Appalachian region, its eastern border. The sandstones 

 and conglomerates of northern Vermont, Pennsylvania, Virginia, 

 and Tennessee, afford proof of shallow waters and emerged flats 

 like those of New York and the States west. There are more 

 shales ; but the interstratified sandstones still show that there was 

 no deep ocean along the Appalachian region. Moreover, in Pennsyl- 

 vania, Virginia, and Tennessee there are worm-burrows in the 

 sandstones, as in New York. Thus, we make out a region of shal- 

 low waters and exposed sands along the Appalachian region, during 

 a part at least of the Potsdam epoch. If, then, there were shallow 

 waters on this eastern border and along the north, we may feel 

 confident that there were no deep seas over the interior basin, 

 although it is probable that the waters deepened from the northern 

 coast-lines southward ; and, as the Appalachians follow the general 

 course of the existing coast, there is ground for full assurance that 

 the general shape of the future continent was marked out in that 

 early period. 



(3.) Eastern border. — On the eastern border of the continent, in 

 Newfoundland, and at Belle Isle, there is similar evidence of shallow 

 waters or emerging sands. Besides the occurrence of a considerable 

 thickness of sandstone, the rock at Belle Isle contains the borings 

 of the Scolithus. There are also fossils ; and above the sandstone 

 occur beds of fossiliferous limestone, which may be additional 

 proof that the seas were not very deep. 



(4.) Appalachian region. — But the great thickness of the deposits 

 along the Appalachian region gives further evidence that the future 

 history of the Atlantic border was already foreshadowed in Primor- 

 dial time. 



Even the shales which abound in these Appalachian beds are no 

 proof of deep seas, as such deposits form along coasts and not in 

 the depths of an ocean. A few hundred feet, or a thousand at 

 most, are all the depth that can be deemed probable for the form- 

 ation of any shale; This is proved by the facts in existing seas. 

 They may originate in deeper waters ; but such cases are exceptions. 

 Moreover, at Georgia, in Vermont, where the rocks are shales, the 



