CHANGES OF POSITION AND LEVEL. 723 



Laurentian period) of the Azoic age ; (2) probably, the close of the 

 Lower Silurian, for part of the Green Mountains ; (3) the close of the 

 Palaeozoic era, for the greater part of the Appalachian region, 

 between Labrador and Alabama. It appears, then, that the ten- 

 sion within the crust continued accumulating through long inter- 

 vals, before it reached that degree which was sufficient to bring on 

 an epoch of plication, uplift, and metamorphism. No one will 

 pretend to count the thousands of centuries between the Azoic era, 

 or the close of the Lower Silurian, and the close of the Palaeozoic 

 era. In Europe, and probably in western America, the intervals 

 were less ; moreover, great uplifts, plications, and metamorphism 

 took place in these regions after the Palaeozoic ; but in every case 

 the period during which tension was accumulating, preparatory for 

 the epoch of disturbance, was a long one ; for the epochs of the 

 elevation of mountains, even in Europe, are but few in number in 

 the whole course of past time. 



7. Oscillations and minor uplifts. — But during this period of accu- 

 mulating tension other and minor effects were apparent. Oscilla- 

 tions of the crust, causing changes of level, were going on unceas- 

 ingly, and they are yet in progress. The alternations of level 

 through the Palaeozoic in North America require no other explana- 

 tion. They were part of the indications of that living and growing 

 force which was to exhibit its grandest results after the Carboni- 

 ferous age had ended. 



8. The water-line of the ocean liable to variations from oceanic subsidences. 

 — As all parts of the earth, oceanic as well as continental, must 

 have participated in the changes of level, the water-level was ever 

 fluctuating like the land-level ; and hence it is not safe to measure 

 the latter always by the former, as is too commonly done. Many 

 of the apparent elevations may have been due to a deepening of 

 the oceanic basin, — which has nearly three times the area of the 

 land (p. 10), — and some of its apparent subsidences may have been 

 caused by an elevation of its bottom. It is probable that at least 

 1000 feet of the height of the continents — the average height of 

 the land of the globe — has arisen from the increase in the depth 

 of the ocean which took place during the successive Palaeozoic, 

 Mesozoic, and Cenozoic eras. 



9. Mountains small elevations compared with the extent of the globe. — It 

 should be remembered, in this connection, that mountains are 

 relatively to the size of the earth but little ridgelets on its surface. 

 A chain 10,000 feet high would stand up only one-tenth of an inch 

 on a globe 110 feet in circumference, or 35 feet in diameter, — as 

 large as many a capacious house ; and one-hundredth of an inch would 



