204 



ELEVATION OF LAND. 



"Ill order," says Mr. Lyell, "to give some idea 

 of the enormous amount of change which this con- 

 vulsion may have occasioned, let us assume that 

 the extent of country moved was correctly esti- 

 mated at 100,000 square miles; an extent ju^t equal 

 to half the area of France, or about five sixths of 

 the area of Great Britain and Ireland. If we sup- 

 pose the elevation to have been only three feet, on 

 an average, it will be seen that the mass of rock 

 added to the continent of America by the move- 

 ment, or, in other words, the mass previously be- 

 low the level of the sea, and,*after the shocks, per- 

 manently above it, must have contained 57 cubic 

 miles in bulk (or about as high as Mtn?.), with a cir- 

 cumference at the base of nearly 33 miles." Mr. 

 Lyell calculates that the mass of rock elevated must 

 have been, if two miles thick, 200,000 cubic miles 

 in length, and have exceeded in weight 363 million 

 of the great pyramids of Egypt. 



In the year 1819 a violent earthquake was expe- 

 rienced at Cutch, in the Delta of the Indus, which 

 caused the sea to rush in by the Eastern mouth of 

 the Indus, and convert a tract of land 2000 square 

 miles in area into an inland sea. At the same time, 

 also, a tract of country 50 miles in length and 16 

 in breadth was elevated 10 feet, comprising an area 

 of 7000 square miles, equal to about one fourth of 

 Ireland. 



The motion of the ground produced by earth- 

 quakes is not always the same ; sometimes resem- 

 bling the undulating motion of a heavy swell at sea, 

 though much quicker, and being at others tremu- 

 lous, as if some force shook the earth violently in 

 one spot. The former of these is far more danger- 

 ous, as it forces walls and buildings olF their centres 

 of gravity, crushing whatever may be beneath them. 



Earthquakes not only cause an elevation, but 

 also a subsidence of the land. Thus, in the year 

 541, Pompeiopolis was swallowed up ; in 867, Mount 



