THE EXTRUSIVE PROCESSES. 629 



Extrusions seem rather more common with faulted ranges where 

 crushing is less notable and where surface tension replaces com- 

 pression. 



Hypothesis 6. Lavas assigned to melting by depression. — It is ob- 

 served that in certain regions great thicknesses of sediments have accu- 

 mulated by the slow settling of the crust below, and as these sediments 

 obstruct the outward flow of heat while the lower beds settle nearer 

 to the interior source of heat, it is conceived that they become heated 

 below and, being saturated with water, take on aqueo-igneous fusion 

 and rise as lavas, well supplied with internal gas and steam from the 

 water and volatile constituents that were entrapped and carried doT\ii 

 with them. The question obviously arises whether such depression 

 is sufficient to give the temperatures the lavas show, and whether volcanic 

 action is confined to such areas of depression and deep sedimentation. 

 At the highest credible estimates — which are none the less to be taken 

 with reserve — the post-Archean sediments rarely reach five or six miles 

 in thickness at any giA^en point, and probably never exceed ten or twelve, 

 while twenty or thirty miles is the computed depth required for the 

 acquisition of the temperatures the lavas actually possess. If the Archean 

 terranes be included among the sedimentaries, the thickness may be 

 adequate, but what then of the Archean vulcanism, which much sur- 

 passes that of later times, and the other early extrusions before the 

 sediments were thick; and what of the moon, where there are probably 

 no sediments at all? 



Besides, it is not at all clear that the distribution of vulcanism is 

 specially related to that of thick sediments, as it should be if this hypoth- 

 esis were the true one. There are many volcanoes in the heart of the 

 great oceans where sedimentation is now inappreciable, and probably 

 has been in all past periods. 



Hypothesis 7. Vulcanism assigned to the outflow of deep-seated heat. 

 — If the earth grew up by slow accessions of meteoroidal or planetesimal 

 matter, in a manner to be more fully set forth in the discussion of the 

 origin of the earth, and if its interior heat be due chiefly to compression 

 by its own gravity, the internal temperature would be originally dis- 

 tributed according to the degree of compression, and this would depend 

 on the intensity of the internal pressure. This can be approximately 

 computed, and is shown in the diagram on page 563, where this subject 

 has been treated. On not improbable assumptions regarding the 



