304 Rev. 0. Fisher — On Dynamo-Mctamorpltism. 



has been that a quantity G A B H of rock has been forced up to a 

 height io above its original level A B. 



Divide the distance between B C and D F into w and x ; to 

 being equal to the height w, to which A B has been lifted. Let IV 

 be the weight of the " cover." Then the work done by P upon 

 the original mass of material has been P (w -\- x) ; and the work 

 done by the material in raising the cover has been W w. Their 

 difference is the work expended upon the mass G D C H, and it is 

 P (w + x ) — W io : or, work = (P — W) w -f- Px. 



Now since the volume B G equals the volume B K, it is obvious 

 that the volume K E has disappeared during the compression. 

 Hence, P x expresses the work of condensation. Consequently 

 (P — W) w must express the work of shearing expended upon the 

 rock. 



We must now inquire into what forms of energy the work has 

 been converted. And first of the work of shearing. 



This is converted, 



(1) Into the potential energy implied in the raising of the centre 

 of gravity of the mass through a height \ w. 



(2) It is employed in bending and breaking the rock, and over- 

 coming friction. Since this part of the energy is not reconvertible 

 into mechanical work, it must take the molecular forms of heat and 

 chemical action, the effects being most concentrated where the 

 mechanical action has been greatest. 



The work of condensation P x goes wholly to increase the results 

 of (2) ; but the distribution of the effects will be more uniform than 

 of those caused by the shearing. 



To estimate the effect of the "cover" we observe that, as the cover 

 is increased, W is increased, and of course P must also be increased, 

 because it will require a greater force to lift a greater mass of over- 

 lying rock. But if instead of rock we had a liquid mass, we see 

 that if P would lift W, P + Z would equally lift W+ Z, while, 

 although the two pressures are equally increased, their difference 

 remains the same. It seems probable, therefore, that, if the rigidity 

 of the rock were to remain unaltered, P — W would not be affected 

 by a change in the quantity of cover. Hence, the work of a given 

 amount of shearing expended upon the mass will not be affected by 

 the amount of cover, except in so far as additional pressure increases, 

 by closeness, the rigidity and frictional resistance of the substance, 

 and so, when W is increased, requires P to increase at a more rapid 

 rate than W. This appears to be a reason why dynamo-meta- 

 morphism may be greater at greater depths. 



Mr. Harker seems to be of opinion, " since energy not lost by 

 conduction must be absorbed in the production of mineralogical 

 changes," that, where there is little hindrance to conduction, such 

 changes will be smaller. But this conclusion does not follow 

 unless heat longer retained is rendered conducive to mineralogical 

 chemical reactions by the consequent increase of temperature. 

 Appropriate elevation of temperature will no doubt help to promote 

 chemical reactions. But in so doing does the heat disappear as heat ? 



