192 The Field of Geology — McGee. 



irregularity of the terrestrial surface. Such movements have been 

 collectively designated displacement or diastrophism; but in the 

 present connection at least they may be grouped as deformation^ 

 and the quality of the movement may be characterized as diastatic. 

 The movements are partly vertical (though there is always a hor- 

 izontal element) and are most easily measured from a fixed datum 

 plane, such as sea level, and they are therefore commonly separated 

 into elevation and depression. 



The second great category of movements comprises the various 

 processes of aqueous erosion and deposition initiated by the primary 

 deformation of the terrestrial surface. By these processes moun- 

 tains and continents are degraded, and sea and lakes are filled with 

 their debris; these processes, too, have been in active operation 

 from the dawn of geologic history to the present; and they ever 

 tend to restore the geoid by obliterating the irregularities of the 

 terrestrial surface proiuced by diastatic movement. The processes 

 may be collectively called gradation; and the antagonistic oper- 

 ations comprehended under the term are designated respectively 

 degradation and deposition. 



A subordinate class of processes by which the rocks of the 

 «arth are formed or afEected is the extravasation of lavas and other 

 volcanic matter from beneath the surface, and the outflow of sub- 

 terranean waters containing minerals in solut on, together with 

 the consequent collapse of cavities and other movements within the 

 crust of the earth. These processes have been in operation 

 throughout geologic time, though perhaps with dimin'shiug ac- 

 tivity; they have added materially to the superficial strata of the 

 earth; and they have modified the geoid not only by addition with- 

 out but by commensurate loss within and consequent deformation 

 or structural alteration. The operations are commonly compre- 

 hended under the name extravasation \ and, like the other primary 

 categories, this comprises two subordinate classes of processes of 

 antagonistic tendency, which, simply for the purpose of fixing 

 their relations, may be called efflux and collapse. The vibra'ory 

 movements of seisniism probably result from both deformation and 

 vulcanism under certain conditions. 



The second subordinate category of processes by which the 

 rocks themselves and the operatious of the second great category 

 of geologic processes are modified, comprises the chemic and 

 chemico-mechanical alterations in constitution and structure of 



