lO 



PRINCIPLES OF STRATIGRAPHY 



that these differences are of fmidaniental value, aiul tliat hence tlie 

 great relief features of the earth's surface have been persistent 

 since the earliest time (Fig. 2). Where the isostatic equilibrium is 

 disturbed by erosion of the higher less dense masses, and by the 

 transference of the product to the denser block, a compensatory 

 deep-seated transference of material by flow must occur, from the 

 denser to the lighter, which is accompanied by a sinking of the 

 upper part and surface of the denser block and a corresponding 

 rise above the level of compensation of the upper part and surface 

 of the less dense block. (Hayford-13 :7pp.) 



Thickness of the Earth's Crust. In considering the top of 

 the lithosphere as representing the surfaces of a series of elevated 

 and depressed crustal blocks, we naturally assume that the thick- 



sun f,4ce 



OC€AN BOTTOM 



y 



DEPTH X 



OCPTm or COMPENS/lTlOIV 



COLUMNi A COLUMN) B 



Fig. 3. Diagram illustrating the conception of isostatic equilibrium and its 

 adjustment with change in surface. (After Hayford.) 



ness of the crust is measured by the height of these blocks. Since 

 they are considered to be in a state of isostasy, it follows that, if 

 these masses were divided into prismatic columns of equal basal 

 area, the pressure due to gravity at the bases of these columns 

 would be the same. The depth at which this state of equilibrium 

 is found is the depth of compensation. From calculations made bv 

 the Coast and Geodetic Survey on numerous observations scattered 

 over the United States, the conclusion has been drawn that the 

 most probable depth of compensation is 76 miles, and that it is 

 practically certain that it is not less than 62 nor more than 87 miles. 

 (Hayford-i2 t//-/^"; 13:^00.) This assumes a uniform position of 

 the level of compensation with reference to depth. In Fig. 3, 

 adapted from Hayford, columns A and B have been assumed to 

 contain equal masses. There is complete isostatic equilibrium and 



