SUMMARY OF IGNEOUS AGENCIES 69 



must be quite independent of isostatic adjustment, because they 

 increase or diminish those inequalities. 



The most recent investigations are unfavourable to this view 

 of isostasy, and go to show that the crust is not so sensitive to 

 changes of load. While the major inequalities, the great conti- 

 nental platforms and oceanic depressions, are not improbably due 

 to differences of density in the crust, minor inequalities, like hills 

 and mountains, are apparently sustained by the rigidity of the crust. 



Summary. — The study of the subterranean, or igneous, agen- 

 cies has proved to be very unsatisfactory in the way of explaining 

 the phenomena and referring them to the operation of understood 

 physical agents, because so little is really known and so much 

 remains to be discovered. Nevertheless, we have learned much 

 that is of great importance in geological reasoning. We have 

 seen that the earth contains within itself a great store of energy, 

 and that its interior, in whatever physical state that may be, is 

 highly heated, and possesses great quantities of material which is 

 either actually or potentially molten, and is permeated with super- 

 heated steam and other gases. This molten material is often 

 forced upward, and is either poured out at the surface, or fills up 

 fissures and cavities in the rocks, or pushes its way between them. 

 Cooling under various circumstances, the molten masses consoli- 

 date into a great variety of characteristic rocks, frothy, glassy, or 

 crystalline. Explosive discharges of steam blow the melted rock 

 into fragments of all grades of fineness, and these fragments like- 

 wise accumulate either on the land or under water, and form 

 rocks, the nature and origin, of which may be readily recognized. 



We have further seen that the operation of these subterranean 

 forces produces shocks and jars in the interior, which are propa- 

 gated to the surface as earthquakes, and there bring about per- 

 manent changes, associated with the Assuring and dislocation of 

 the rocks, landslips, alteration in the course of rivers, formation 

 of lakes, and the like. The frequency of earthquakes, their wide 

 geographical range, and the constant tremor of the ground 

 detected by delicate instruments, led us to infer that the crust 

 of the earth is decidedly unstable. 



