724 DYNAMICAL GEOLOGY. 



correspond on such a globe to the mean height of the continents. 

 If the Eocky Mountains on a globe of this size were given their 

 actual slope (equal on the east side to two or three feet in 5000 

 feet of length), they would be hardly recognizable. The highest 

 peaks of the Appalachians would have a height of only one-sixteenth 

 of an inch, and the highest of the globe, of only three-tenths of an inch. 

 A change of level in the crust of 100 feet, which might, in the ear- 

 lier geological ages, have lifted a large part of a continent out of 

 the sea, would be represented by one-thousandth of an inch on the 

 same globe. The movements for such effects would relatively, 

 therefore, be exceedingly small. Considering the length of time 

 which must have elapsed since the crust of the globe was first formed 

 and through which contraction has been effecting its changes, and 

 the vastness of the force that would thus be produced in the crust 

 of a globe 25,000 miles in circumference, it may rather occasion 

 surprise that the highest summits stand only 30,000 feet above the 

 ocean's level, and less than 100,000 feet above the lowest depths of 

 the oceanic basin. 



10. Courses of elevations in a region the same in different periods. — The 

 elevations and strike of the rocks in northern New York, which 

 date from the Azoic age, — the first emergence of the Green Moun- 

 tains, dating from the close of the Lower Silurian,— the plications, 

 elevations, and metamorphism of the larger part of New England, 

 dating from the close of the Palaeozoic era, — the formation of the 

 trap ridges of the Connecticut Eiver valley, dating from the middle 

 Mesozoic era, — have the same general direction. The Mesozoic 

 trap ridges and the plications and uplifts of the Appalachians in 

 Pennsylvania are also nearly parallel ; and the same is true of the 

 corresponding elevations in Virginia. These few examples are 

 sufficient to illustrate the principle stated. 



11. Courses of elevations in a region not the same in different periods. — 

 Europe contains many examples of this diversity of direction in 

 the same region : on page 533 a case of this kind in the Alps is 

 mentioned. It seems natural that the elevating force should vary 

 somewhat its direction with the progress of time, or, if remaining 

 the same, that it should encounter a difference of resistance which 

 should lead to a result unlike those in former periods. 



12. Courses of elevations different in the same period. — The Mesozoic 

 trap ridges and sandstone of Nova Scotia trend nearly northeast, 

 those of the Connecticut valley north-by-east, those of Pennsylvania 

 east-northeast, and those of Virginia northeast-by-north ; and yet 

 there is every reason to believe that they belong to the same period 

 of origin. The Appalachian chain varies much in directions 



