24 PHYSIOGRAPHIC GEOLOGY. 



the mountain regions than by any special altitude. Tlie surface is usually 

 undulating, and often liilly. The great interior region of the North Ameri- 

 can continent, including the Mississippi valley, is an example of an interior 

 plain ; also the plains of the Amazon ; the pampas of La Plata ; the lower 

 lands of Europe and Asia. Frequently the surface rises gradually into the 

 bordering mountain-declivities, as in the case of the Mississippi plains and 

 the Eocky Mountain slope. Broad, low plains between mountain ranges and 

 the seashore are called coastal plains. Along the eastern border of North 

 America from New Jersey southward, the coastal plains are broad and have 

 navigable streams. Next west is a region of more uneven and rocky country 

 with rapid streams — the Piedmont region, which extends to the Appalachian 

 region, or that of the mountains. 



A mountain is either a single peak, as Mount Etna, Mount Washington, 

 Mount Blanc ; or a ridge ; or a series of ridges, sometimes grouped in many, 

 more or less parallel, lines. 



A inountain range consists of a series of ridges closely related in position, 

 direction, and origin : as in the Appalachian ranges, the Wasatch, the 

 Sierra Nevada. A sierra is, in Spanish, the name of a ridge, or group of 

 ridges, of serrated or irregular outline. 



A mo^mtain system consists of two or more mountain ranges, of the same 

 period of origin, belonging to a common region of elevation, and generally 

 either parallel or in consecutive lines, or consecutive curves, with often 

 inferior transverse lines of heights. A mountain chain consists of two or 

 more mountain-systems of different periods of origin, in the same part of a 

 continent. The oldest of the mountain ranges in a chain is called the protaxis 

 — so named from the Greek iov first and axis (see the map of the Archaean 

 areas on page 443). The other ranges are usually parallel to the protaxis, 

 and may, or may not, have greater height. The Appalachian Chain ex- 

 tends from Canada to Alabama, and comprises (1) the protaxis, represented 

 by the Highlands of New Jersey and Putnam County, New York, and their 

 continuation northward interruptedly along the eastern half of the Green 

 Mountains into Canada, and southward, as a narrow, interrupted area, 

 through Pennsylvania, and a very broad area through Virginia, to Georgia j 

 (2) the Taconic Range, along the borders of New England and New York to 

 New Jersey and beyond ; and (3) the Appalachian Range. 



The Rocky Mountains also have a protaxis, with approximately paral- 

 lel ranges of later formation. This protaxis is the " Front Range " in Colo- 

 rado, nearly 1000 miles from the Pacific coast, making the Pacific border 

 region in this part very wide. But to the north, in Montana and Wyoming, 

 the protaxis makes a westward bend of 250 miles, and then resumes a north- 

 westward course and continues to' the parallel of 521°, and is represented 

 beyond this in isolated ridges ; consequently the Pacific border region of 

 British America is relatively narrow. The line to the north of the United 

 States appears to be represented to the south in the Archaean axis of the 

 Wasatch and some other similar ridges. The very large area of the Pacific 



