240 GENERALIZATIONS. 



deposited in water might sink to great depths, and, having been 

 altered by the high temperatures, might be again thrown out in 

 a metamorphic form. Most geologists of our time are of opinion 

 that in this manner the mountains have been formed. 



But the Neptunists advance a different theory. They deny 

 the existence of the fiery fluid nucleus, and derive the higher 

 temperature of the interior of the earth from chemical processes 

 which there take place and produce heat. They assert that 

 changes are constantly going on in the rocks, and that the 

 materials never rest, entering into one combination and modi- 

 fying it, so that other forms are produced. The rocks may 

 lose substances (as by erosion), and will then crumble and 

 become fissured; or they may become associated with new sub- 

 stances conveyed to them by water ; or they may pass into the 

 crystalline state and thus enlarge considerably in volume ; and 

 when this takes place the masses will increase in magnitude, 

 burst out of the depths, a#d break through the crust above 

 them. The cause of upheaval must therefore be sought in slow 

 and gradual changes of the rock-masses, which in augmenting 

 their volume produce mountains. 



Such processes do take place in nature, and may therefore 

 have had a share in the formation of the Swiss mountains ; but 

 these changes do not suffice to explain a whole series of pheno- 

 mena, such as the spherical form of the earth, the increase of 

 heat towards its centre, the eruption of volcanoes, or the higher 

 and more uniform temperature of the early periods of the earth/ s 

 history. 



Changes have been wrought in the configuration of the ground 

 by upheavals, which have given a great variety to the forms- of 

 mountains and valleys, and which merit consideration. 



When pressure is exerted from beneath upon a horizontal 

 stratum, the latter will arch upwards so long as such a move- 

 ment is permitted by the flexibility of the bed. The effect of 

 this pressure will be manifested at the surface by an undulating 

 line, that is to say by arched portions alternating with depres- 

 sions, the slopes of which are more or less gentle. An idea of 

 the pressure may be given by a piece of paper or cloth submitted 

 to a similar movement. If the pressure be exerted upon several 

 superimposed beds of rocks, and the strata submit to elevation 



