THICKNESS OF STRATA 333 



A Mountain System is made up of a number of parallel or 

 consecutive ranges, formed in separate geosynclines, but of ap- 

 proximately similar dates of upheaval. The Appalachian system 

 comprises the Appalachian range, running from New York to 

 Georgia, the Acadian range in Nova Scotia and New Brunswick, 

 and the Ouachita range in Arkansas. Each of these ranges was 

 formed in a different geosynclinal, but at the same geological 

 date, and they are consecutive, having a common direction. 



A Mountain Chain comprises two or more systems in the same 

 general region of elevation, but of different dates of origin. The 

 Appalachian chain includes the Appalachian system, the Blue 

 Ridge, the Highlands of New Jersey and the Hudson, a system of 

 different date, and the Taconic system of western New England, 

 which was not formed at the same time as either of the others. 



A Cordillera consists of several mountain chains in the same 

 part of the continent. Thus, the chains of the Rocky Mountains, 

 Sierra Nevada, Coast Range, and their prolongations in Canada, 

 together make up the Rocky Mountain or Western Cordillera. 



From these definitions it will appear that the mountain range 

 has a unity of structure and origin which fits it especially for 

 study. If the structure and history of the ranges be understood, 

 the systems and chains will offer little additional difficulty. 



A mountain range (disregarding, for the present, certain excep- 

 tional cases) consists of a very thick mass of strata, which are 

 much thicker in the mountains than the same strata in the 

 adjoining plains. In the Appalachian range, for example, the 

 stratified rucks are nearly 40,000 feet thick, but on tracing 

 the same series of beds, westward into the Mississippi valley, 

 they are found to become very much thinner, hardly exceeding 

 one-tenth of the thickness in the mountains. This immense thick- 

 ness of the component strata is not peculiar to the Appalachians 

 but reappears in the typical mountain ranges everywhere : the 

 Wasatch range has 31,000 feet of strata, the Coast Range 30,000 

 feet, the Alps 50,000 feet, etc. The thick series of strata which 

 make up a mountain chain are usually conformable throughout ; 

 deposition was, for the most part, continuous, and there was 



