112 



AUSTRALASIAN ANTARCTIC EXPEDITION. 



in Van Hise's zone of katamorphism. External energy has been impressed upon the 

 system, and olivine and pyroxene have passed over into serpentine, liberating a large 

 quantity of heat and molecular energy. The accompanying figures, quoted from Van 

 Hise's tables,* show that this type of alteration in peridotites is accompanied by the 

 formation of minerals of lower specific gravity and larger molecular volume. 



According to Van Hise's interpretations of the reactions, the following figures express 

 the increase in volume of the individual systems : 



In all cases except the last, which Flett has only stated as a possibility in the Lizard 

 serpentine, there is increase in volume and liberation of heat. Hence, in a large mass 

 of rock like the Lizard serpentine, there will be enormous expanding forces exerted on 

 the enveloping rocks and on any dyke sheets that happen to traverse the serpentine. 

 These forces will act as compressive stresses approximately normal to the dyke plane 

 whatever may happen to be its direction. The serpentinisation thus causes the squeezing 

 of the dyke rock. Large quantities of heat are liberated at the same time, and the 

 compression is met by the molecular rearrangement of the constituents with the 

 development of those forms and shapes which are most stable against the imposed 



* " Treatise on iletamorphism," p. 196. 



