1856.] Physical Geography of Nmth America, 171 



does not entirely separate this long, nearly continuous mountain 

 system. A depression of the continent to a depth of only 300 feet 

 would let the ocean through from the Gulf of St. Lawrence to the 

 Bay of New York, and interpose a strait nearly as broad as that 

 between England and the European continent, between the main 

 land of North America and the even now half-insulated district of 

 New England, New Brunswick, and Nova Scotia. That this great 

 transverse depression of the surface, now uniting the estuaries of 

 the St. Lawrence and the Hudson, was once thus actually submer- 

 ged is proved by indisputable geological monuments, consisting of 

 oceanic clays containing marine pleistocene tertiary fossils. 



The North-eastern section of the Appalachians contains as its 

 principal chain the Green Mountains, a long belt of parallel and 

 swelling ridges commencing in the Notre Dame range south of the 

 St. Lawrence, in Lower Canada, and terminating at the Schuylkill 

 River in Pennsylvania. These mountains at the Hudson, under the 

 name of the Highlands, are cut to their base by that tidal river. In 

 Vermont their rounded summits are 4000 feet high, but on the 

 Hudson they nowhere exceed 1 600 feet. The White Mountains of 

 New Hampshire, and the Adirondac group in the north-east corner 

 of New York, are lofty outlying masses, subsidiary to the Appala- 

 chians. They consist chiefly of the crystalline rocks. Their highest 

 peaks exceed 5000 feet, and that of Mount Washington, the loftiest 

 east of the Rocky INIountains, is 6428 feet above the sea level. 



South-western Appalachians. — From the Hudson and Mohawk 

 to middle Alabama extends the other great division of the Appa- 

 lachian chain, marked by an extraordinary parallelism, length, 

 narrowness, steepness, and evenness of its numerous ridges. Its 

 south-eastern belt, the most undulating and picturesque, is the Blue 

 Ridge of Virginia, and the Smoky or Unaka Mountain range of 

 Carolina, Georgia, and Tennessee ; it is nearly identical in geological 

 structure with the Green Mountains, and resembles them in contour 

 and scenery. Its highest crests, which are in North Carolina and 

 Tennessee, reach to 4000 and 5000 feet above the sea. The rocks 

 of the Blue Ridge and Green Mountain chains are chiefly ancient 

 metamorphic strata, as gneiss, talcoze, and chloritic schists, and highly 

 altered argillaceous and sandy strata low in the palaeozoic system, 

 including, indeed, some of the oldest fossiliferous formations : these 

 stratified masses are penetrated by igneous veins and dykes. 



To the north-west of the Blue Ridge, and separated from it by 

 a long continuous plain called the Great Appalachian Valley, runs 

 the parallel range of the middle Appalachian belt, traceable from 

 the St. Lawrence to Alabama. This central zone of mountains 

 consists of long narrow level ridges, divided by narrow longitudinal 

 valleys, and cut to their bases by sharp deep ravines permitting the 

 passage of the rivers. These ridges, caused by excessive erosion of 

 the parallel waves of the strata of which the central Appalachians 

 consist, are linked into groups of several parallel crests, some of 



