28 National OeograjpJiic Magazine. 



The initial geologic movements (so far as may be inferred 

 from the present condition of the earth) were distortions or dis- 

 placements of the solid or solidifying terrestrial crust, occurring 

 in such manner as to produce irregularities of surface. These 

 are the movements involved in mountain growth and in the up- 

 heavel of continents. They have been in operation from the 

 earliest known eons to the present time, and their tendency is 

 ever to deform the geoid and produce irregularity of the terrestrial 

 surface. The movements have been called collectively " displace- 

 ment" and " diastrophism," but in the present connection they may 

 be classed as diastatic, or, in the substantive form, as deforma- 

 tion. Recent researches, mainly in this country, have indicated 

 that certain diastatic movements are the result of transference of 

 sediment — that areas of loading sink, and ai'eas of unloading rise ; 

 but it is evident that the transference of sediment is itself due to 

 antecedent diastatic movements by which the loaded areas were 

 depressed and the unloaded areas elevated; and the entire cate- 

 gory may accordingly be divided into antecedent and consequent 

 diastatic movements. A partially coincident division may be 

 made into epeirogenic, or continent-making movements (so called 

 by Gilbert), and orogenic, or mountain-making movements. 

 Though there is commonly and perhaps always a horizontal com- 

 ponent in diastatic movement, the more easily measured compo- 

 nent is vertical, and when referred to a fixed datum [e. g. sea level) 

 it is represented by elevation and depression. 



The second great category of geologic processes comprehends 

 the erosion and deposition inaugurated by the initial deformation 

 of the terrestrial surface. By these processes continents and 

 mountains are degraded, and adjacent oceans and lakes lined with 

 their debris. They have been in active operation since the dawn 

 of geologic time, and the processes individually and combined 

 ever tend to restore the geoid by obliterating the relief produced 

 by deformation. The general process, which comprises degra- 

 dation and deposition, may be called gradation. 



The first subordinate category of movements is allied to the 

 first principal category, and comprises, (1) the outflows of lavas, 

 the formation of dykes, the extravasation of mineral substances 

 in solution, etc., (2) the consequent particle and mass movements 

 within the crust of the earth, and (3) the infiltration of minerals 

 in solution, sublimation, etc., — in short, the modification of the 

 earth's exterior directly and indirectly through particle movements 

 induced by the condition of the interior. These processes have 



