412 Captain F. W. Hutton on the Phenomena of the 



observed facts with theoretical deductions; for in order to do 

 this we must know the thickness of all the formations that have 

 ever overlain the district, and the number and amount of the 

 oscillations they have undergone; and even then there might be 

 various causes, such as alterations in temperature, which would 

 affect the result. 



I have now explained the Herschel-Babbage theory in its 

 simplest form ; but in nature this simplicity would seldom or 

 never exist. Complications would arise (1) from changes in phy- 

 sical geography causing changes in the surface-temperature, 

 according to the theory of Sir C. Lyell, (2) from the movements 

 of the isogeothermals being often oblique, (8) from the different 

 degrees of fusibility of rocks, (4) their different conducting- 

 power and (5) rates of expansion, and (6) their varying degrees 

 of porosity, and also (7) from new fluctuations of temperature 

 commencing before the old ones had terminated. The two great 

 motive powers, alteration in volume and increase of weight, 

 would also sometimes combine, and at other times interfere with 

 one another like cross waves on the surface of the sea, and would 

 thus give rise to the great irregularities that we see in nature. 



I will now shortly consider from the stand-point given by this 

 theory some of the principal geological phenomena that have 

 not yet been mentioned. 



Volcanoes require an upward pressure of the superheated rocks 

 and sufficient tension in the upper rigid crust to form fissures ; 

 they ought therefore to be situated on land rising from the up- 

 ward pressure derived from adjacent subsiding areas, which are 

 sinking either from an increase of pressure or from a decrease 

 in temperature. This can certainly be shown to be the case 

 with the volcanoes of South America, the Pacific*, Indian Archi- 

 pelago, and Iceland; but it is not so easy to account for Etna 

 and Vesuvius. 



Faults can only be formed when the rocks are undergoing 

 tension, as it is mechanically impossible that compression should 

 force up a wedge of rock ; for it would crush or bend before it 

 would move. After a district has been folded by compression 

 the rocks would not, on tension following, go back into their 

 former position, but fissures would be formed through them, and 

 those wedge-like masses that have their point turned downwards 

 would descend by their own weight ; or if the land be stretched 

 by pressure from below, those with their points turned upwards 

 will be driven upwards. 



Cleavage. — Mr. Sharpe has remarked (Quart. Journ. Geol. 

 Soc. vol. lii. p. 104) that "there are reasons for thinking that 



* Coral islands would not cause elevation, as they are not connected 

 with one another. 



