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XIII. On some Results of the Earth's Contraction from Cooling, 

 including a discussion of the Origin of Mountains. By James 

 D. Dana. 



[Continued from p. 54.] 



5. Mountain-making slow work. 



TO obtain an adequate idea of the way in which lateral pres- 

 sure has worked, it is necessary to remember that 

 mountain-elevation has taken place after immensely long periods 

 of quiet and gentle oscillations. After the beginning of the 

 Primordial, the first period of disturbance in North America of 

 special note was that at the close of the Lower Silurian, in 

 which the Green Mountains were finished ; and if time from the 

 beginning of the Silurian to the present included only fifty 

 millions of years — which most geologists of the present day 

 would consider much too small an estimate — the interval be- 

 tween the beginning of the Primordial and the uplifts and 

 metamorphism of the Green Mountains was at least ten mil- 

 lions of years. The next epoch of great disturbance in the same 

 Appalachian region was that at the close of the Carboniferous 

 era, in which the Alleghanies were folded up — by the above 

 estimate of the length of time, thirty-five millions of years * 

 after the commencement of the Silurian; so that the Appala- 

 chians were at least thirty-five millions of years in making, the 

 preparatory subsidence having begun as early as the beginning 

 of the Silurian. The next on the Atlantic border was that of 

 the displacements of the Connecticut-river sandstone, and the 

 accompanying igneous ejections, which occurred before the 

 Cretaceous era — at least seven millions of years, on the above 

 estimate of the length of time, after the Appalachian revolution. 

 Thus the lateral pressure resulting from the earth's contraction 

 required an exceedingly long era in order to accumulate force 

 sufficiently to produce a general yielding and plication or dis- 

 placement of the beds, and start off a new range of prominent 

 elevations over the earth's crust. 



6. System in the mountain-making movements on the opposite 

 borders of the North American Continent and over the Oceanic 

 areas. 



A summary of the general system of movements and moun- 

 tain-making on the opposite borders of the continent, and over 

 the oceanic areas, will, I think, render it apparent that the views 

 here sustained have a broad foundation. 



I omit any special reference to the Archaean elevations, and 



* These estimates of the relative lengths of ages are based on the maxi- 

 mum thickness of their rocks — very uncertain data, but the best we have. 



