12 PRINCIPLES OF STRATIGRAPHY 



tion may rise. It thus appears that the adjustments within the 

 earth's crust and tlie forces responsible for the geological changes 

 recognizable on the surface of the earth are confined to the upper 

 76 miles of the earth's mass, or about 1/53 of the radius. This we 

 may, therefore, regard as the crust of the earth, bearing in mind, 

 however, that there is no marked line of separation between the 

 crust and the subcrustal part. It is within this crust, and chiefly 

 within its upper part, that we find the seat of vulcanism ; that the 

 minor disturbances recorded as earthquakes, etc., occur ; that the 

 ground water circulates, and that the crushing and flowage of rocks 

 take place, and it is by changes in the crust as thus defined that 

 the rise and fall of land masses and sea bottoms take place. It is 

 of course possible that the source of some of our basic volcanic 

 rocks is deeper than 75 miles. Thus the density increase in the earth 

 cited below suggests that basalts are derived from depths of 105 to 

 137 miles. 



Material of the Earth's Crust. Only the material of the 

 lighter blocks of the earth's crust, i. e., those constituting the con- 

 tinental masses, is open to observation, and constitutes the "rocks 

 of the earth's crust." The average specific gravity of this material 

 is 2.2 to 3 (average 2.6), while that of the earth as a whole is 

 about 5.6. This difiference is accounted for by assuming that the 

 material constituting the interior of the earth (centrosphere) has 

 a higher specific gravity than the earth as a whole. It is clear that 

 if the continental and suboceanic crustal blocks are in the condition 

 of static equilibrium, the latter must consist of heavier or denser 

 material than that of the known surface. 



Deformation of the Earth's Crust. (Diastrophism.) The 

 larger deformations of the earth's crust consist in the sinking of 

 the suboceanic crustal blocks, or the rise of the continental blocks, 

 or vice versa.- These are designated epeirx)genic (continent-mak- 

 ing.) movements. Minor diastrophic changes result from local 

 warpings. either up or down, from faultings, or foldings of the 

 strata. These are designated as orogenic (mountain-making) 

 movements, resulting in the formation of mountains. Local rising 

 of the land, even though unaccompanied by visible foldings, must be 

 considered as bowing or folding on a large scale, and it often pre- 

 cedes the formation of folded mountains, as shown by the suc- 

 cessive elevations of the Appalachian Old Land (recorded in the 

 successive continental fans), which preceded the folding of the 

 strata at the end of Palzeozoic time. 



IV. THE PYROSPHERE. This is an indefinite region in the 

 lower part of the earth's crust, or below it, and designated as dis- 



