RELATION OF DYXAMIC FORCES TO MOUXTAIX MAKING 395 



to be the compound zone of earthquakes, which is apparently within the 

 zone of more recent ridging in front of the major uplifts. 



Third. Within the second zone are arranged the volcanoes which again 

 fall into the active or extinct types, according to the zones of pressure or 

 relative stability within which they occur. All, however, are of Pleisto- 

 cene age. 



A necessary modification to this statement is that the mountain rings 

 form individual units arranged subparallel to each other and within 

 which these arrangements of earthquakes and of volcanoes occur, not as 

 one line only, but in as many lines as there are unstable island groups. 



iGXEors ly'TRVftJoy^ 



All students of mountainous areas, past and present, have been im- 

 pressed with three facts of worldwide application : 



(a) In the Archeozoic and the lower Proterozoic areas, which are those 

 of intense folding, and in those of the narrower zones of intense folding 

 of Paleozoic age, the amount of igneous intrusions is remarkable. In 

 proportion to the intensity of the metamorphism evidenced, these intru- 

 sions appear to partake of the sill or lenticular form, hing between bed- 

 ding and schistosity planes, especially where the latter are almost coinci- 

 dent with the former. 



(V) In the areas of deep-seated folding of the Cenozoic sediments, and 

 which are exposed today by denudation, a very common occuiTence is the 

 igneous intrusive of the plutonic type as bosses and related forms, with 

 attendant dikes. The Tethyan region exemplifies this well. In regions 

 uplifted at the same time, but not folded, namely, the arcs nearer the 

 continental nuclei, great sheets of basic lavas were extruded without 

 accompaniment of tuff's or explosive features. 



{c) In the great plateaus formed during the Pliocene revival of 

 mountain-making, there was a marked development later of volcanic 

 craters on the inward side of the pressure zones or arcs of Pleistocene age. 



It would seem, therefore, that the formation of great momitains is 

 attended by the introduction of enormous volumes of igneous material 

 within the areas affected by the folding and mountain-making generally. 



The nature of this igneous manifestation is peculiar. Thus in the 

 deepest portions of the crust exposed at the surface today the accumula- 

 tion of igneous material was so great in places as to displace the sedi- 

 ments intruded in great measure. The igneous material appeared to 

 have "sweated*' along the more permeable beds, and to h'ave taken on the 

 form of sills, from which replacements took place in great part by pegma- 



