754 DYNAMICAL GEOLOGY. 



(14.) Mountain-making slow work. — To obtain an adequate idea 

 of the way in which lateral pressure 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 forty-eight millions of years 

 (p. 591), the interval, between the beginning of the Primordial and 

 the uplifts and metamorphism of the Green Mountains, was at least 

 twenty millions of years. The next epoch of mountain-making on 

 the Atlantic Border was after the Devonian in Nova Scotia and New 

 Brunswick ; on the above basis, it occurred thirty millions of years 

 from the beginning of the Primordial. The next epoch of disturb- 

 ance was that at the close of the Carboniferous era, in which the Al- 

 leghanies were folded up ; by the above estimate of the length of 

 time, thirty-six millions of years after the commencement of the Silu- 

 rian ; so that the Alleghanies were at least 36,000,000 of years in 

 making, the preparatory subsidence having begun as early as the be- 

 ginning of the Silurian. The next on the Atlantic Border was that 

 of the displacements of the Triassico-Jurassic Sandstone, and the 

 accompanying igneous ejections, which occurred before the Cretaceous 

 era — at least five 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 time, in order to accumulate force sufficient to produce a general 

 yielding and plication or displacement of the beds, and start off a new 

 range of prominent elevations over the earth's crust. 



IV. CHANGES IN CLIMATE. 



As the cooling of the earth from fusion went forward, the earth's 

 outer temperature finally became climatal ; and although at first ex- 

 cessive in its heat, with the thickening of the crust there was slow 

 amelioration, until a genial climate finally pervaded the surface, when 

 life took its place in the waters. Cooling has ever since gone forward ; 

 but it is supposed that, amid the other causes of change, the heat of 

 the earth's interior has long, perhaps since the Silurian age began, 

 produced little impression on the temperature of the air and waters. 



Yet, while the direct action of the earth's refrigeration may, for 

 some ages past, have had small effect on climate, the indirect effects, 

 or those proceeding from changes of level in the land, increase in its 

 extent and height, and the variations in its distribution about the 

 sphere, have had, as Lyell has shown, vast effect on the climatal phases 



