J. D. Dana on the Origin of Mountains. 349 
believe, physical impossibilities; and LeConte, in his recent 
article, has further enforced this opinion. The third assumes 
that the first 500 feet in depth of sediments would press down 
the crust 500 feet, and so on to the end ; but no reason is given 
why sediments under water should have so immense gravitating 
power, when the crystalline rocks of the Adirondacks, piled to 
a height of some thousands of feet above the water, had a fi 
footing close along side of the subsiding region. 
But while the weight of accumulating sediments will not 
cause subsidence, a slow subsidence of a continental region has 
often been the occasion for thick accumulations of sediments. 
Plies of two lines of elevation, showing that force has con- 
found 
