dt ROYAL SOCIETY OF CANADA 
It should of course be remembered that we are comparing the actual 
coefficient of expansion at 100° C. with the average coefficient of ex- 
pansion over a more or less wide range up to about 100° C. 
It seems reasonable to suppose that the more rapid increase in 
length of the diabase and granite between 350° and 800° C. may be due to 
the different coefficients of expansion of the various crystals composing 
the rock, those having a greater coefficient tending to stretch other parts 
of the rock, until finally at about 350° C. the elastic limit of those parts 
being exceeded, the elongation becomes much more rapid. 
In the same way it would seem that the permanent expansion with 
its consequent weakening of the rock might be explained. As the par- 
ticles of the rock which have a smaller coefficient of expansion are torn 
apart by others having a greater expansion, more or less of a rearrange- 
ment of the loosened particles would naturally follow, so that on cooling, 
instead of completely resuming their original position, they would tend 
to tear apart the crystals having the greater expansion coefficients. 
This would cause both a weakening of the rock and a permanent elonga- 
tion. 
In Figure IX., the permanent expansions have been plotted against 
the number of heatings to show the tendency of the rocks to approach a 
permanent state. It will be recalled that Hallock noted that the per- 
manent expansion of the marble samples upon which he was working 
on “being heated for the first time to 100° C. and allowed to cool did not 
contract to their original length, and the next two or three heatings re- 
sulted in continued, but ever diminishing increments of length at or- 
dinary temperatures.” . It will be seen that a similar result is obtained 
on heating to a much higher temperature. 
In conclusion the author desires to express his thanks to Dr. Barnes 
and Dr. Adams for much help during the progress of the work. 
